/usr/bin/update-manager:json.decoder.JSONDecodeError:/bin/update-manager@118:start_update:start_available:refresh_cache:_get_ua_security_status:load:loads:decode:raw_decode

I reproduced the bug on a fresh 20.04.6 install, just upgraded update-manager, disconnected from internet and started update manager

Failed to connect to authentication server
Check your Internet connection and try again.

[info] A new version is available: 30~20.04
Please run:
    sudo apt-get install ubuntu-advantage-tools
to get the latest version with new features and bug fixes.
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/UpdateManager/Dialogs.py", line 118, in <lambda>
    button.connect("clicked", lambda x: callback())
  File "/usr/lib/python3/dist-packages/UpdateManager/Dialogs.py", line 371, in available
    self.window_main.start_available(error_occurred=True)
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 240, in start_available
    self.refresh_cache()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 458, in refresh_cache
    self._get_ua_security_status()
  File "/usr/lib/python3/dist-packages/UpdateManager/UpdateManager.py", line 284, in _get_ua_security_status
    s = json.load(p.stdout)
  File "/usr/lib/python3.8/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.8/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

ii ubuntu-advantage-tools 27.13.6~20.04.1 amd64 management tools for Ubuntu Pro

$ pro security-status --format=json
Failed to connect to authentication server
Check your Internet connection and try again.

[info] A new version is available: 30~20.04
Please run:
    sudo apt-get install ubuntu-advantage-tools
to get the latest version with new features and bug fixes.

that's resolved after updating ubuntu-advantage-tools to 30~20.04 (the current version)

And some extra though about the issue

- unsure why those systems have an updated update-manager but an old ubuntu-advantage-tools?

- updating our depends on u-d-t would avoid users hitting the problem due to the outdated version

- we should still handle the case where the client doesn't return the a valid json output, either by catching the exception or by backporting the switch to use the proper python api (and verifying that this variant of the code handle the situation better, should be easy to do on an not updated focal system)

The fact that it shouldn't impact updated systems in most cases make it a bit lower importance but we should probably still SRU a fix to the different series

Thank you again for figuring this one out!

> unsure why those systems have an updated update-manager but an old ubuntu-advantage-tools?

I can't think of anything either other than a manual 'apt udpate; apt install update-manager'.

> we should still handle the case where the client doesn't return the a valid json output, either by catching the exception or by backporting the switch to use the proper python api

If it is indifferent, then I'm going with the latter since we already did it for 24.04. The documentation says it raises no exceptions[1].

I'll open the merge request once I have it tested.

[1] https://canonical-ubuntu-pro-client.readthedocs-hosted.com/en/latest/references/api/#u-pro-packages-updates-v1

I had to run 'update-manager --no-update' to get the crash, else Update Manager really would map but display "failed to download repository information" (attachment)...

Did I overlook a step?

It turns out that the API of Ubuntu Advantage Tools 27.13.6 is also affected by the problem we observed in 'ua security-status --format=json', failing the documented expectation of not raising a exception (see log). I can confirm this is no longer a problem for 30.

I think however that it is reasonable move forward assuming that will not be an issue again and that the documented behavior is accurate — as long as version >= 30 is being used, of course.

This doesn't really seem to be a minimal fix, especially since it appears that switching to the Python API but *not* upgrading to u-a-t >= 30 results in a similarly broken experience.

Given the choice between a 1 line diff to debian/control and an architectural change *and also* that same 1-line diff to debian/control it seems clear to me that just the 1-line change is the more appropriate SRU. Is there extra context here that I'm missing that would change this calculus?

If it later turns out that there are bugs that can only reasonably be fixed by switching to the Python API, then that switch can be made for that SRU. I don't think we need to preemptively re-architect this.

@Chris, updating the Depends on u-a-t to >= 30 fixes the obvious error case which can be hit when offline with the old version, but the idea was also to make the update-manager code more robust to parsing error/unexpected output from the pro command and also take the opportunity to move away from parsing the output of a cmdline utility to use a stable API.

Let me know if you think that's enough of a rational for the uploaded changes, maybe with an updated SRU description and testcase, or if you would still prefer to see us uploading the simpler 'bump the depends' fix

This bug was fixed in the package update-manager - 1:24.04.4

---------------
update-manager (1:24.04.4) noble; urgency=low

  * d/control: Depend on ubuntu-advantage-tools >= 30~.
    Fixes edge case whereby the program crashes upon an incomplete
    response from ua security-status (LP: #2049785).
  * The New Release dialog should take precedence over a list of updates dialog
    if the latter would only show Ubuntu Pro updates in an unattached system.
    (LP: #2051115).

 -- Nathan Pratta Teodosio <email address hidden> Mon, 22 Jan 2024 08:17:25 +0100

@Chris, I reverted the client->API change for now, we still probably want to SRU that at some point to be more robust to pro client updates and changes but it doesn't need to be in the way of landing the fixes for the current issue

Hello errors.ubuntu.com, or anyone else affected,

Accepted update-manager into jammy-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/update-manager/1:22.04.19 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-jammy to verification-done-jammy. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-jammy. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Hello errors.ubuntu.com, or anyone else affected,

Accepted update-manager into focal-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/update-manager/1:20.04.10.20 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-focal to verification-done-focal. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-focal. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Hello errors.ubuntu.com, or anyone else affected,

Accepted update-manager into bionic-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/update-manager/1:18.04.11.23 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-bionic to verification-done-bionic. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-bionic. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Hello errors.ubuntu.com, or anyone else affected,

Accepted update-manager into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/update-manager/1:16.04.20 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-xenial to verification-done-xenial. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-xenial. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

All autopkgtests for the newly accepted update-manager (1:20.04.10.20) for focal have finished running.
The following regressions have been reported in tests triggered by the package:

ubuntu-release-upgrader/1:20.04.41 (armhf)

Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-migration/focal/update_excuses.html#update-manager

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

I got a Focal ISO and run it in a virtual machine.

Enabled proposed and now upgrading update-manager forces the installation of a newer ubuntu-advantage-tools:

--->
$ sudo apt install update-manager --verbose-versions
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
   python3-update-manager (1:20.04.10.20)
   ubuntu-advantage-tools (31.1~20.04)
   ubuntu-pro-client (31.1~20.04)
   ubuntu-pro-client-l10n (31.1~20.04)
   update-manager-core (1:20.04.10.20)
The following NEW packages will be installed:
   ubuntu-pro-client (31.1~20.04)
   ubuntu-pro-client-l10n (31.1~20.04)
The following packages will be upgraded:
   python3-update-manager (1:20.04.10.11 => 1:20.04.10.20)
   ubuntu-advantage-tools (27.13.6~20.04.1 => 31.1~20.04)
   update-manager (1:20.04.10.11 => 1:20.04.10.20)
   update-manager-core (1:20.04.10.11 => 1:20.04.10.20)
<---

I confirm then that launching and finishing update-manager without internet connection produces no error any more.

I got a Jammy ISO and run it in a virtual machine.

Enabled proposed and now upgrading update-manager forces the installation of a newer ubuntu-advantage-tools:

--->
$ sudo apt install update-manager --verbose-versions
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
   python3-update-manager (1:22.04.19)
   ubuntu-advantage-tools (31.1~22.04)
   ubuntu-pro-client (31.1~22.04)
   ubuntu-pro-client-l10n (31.1~22.04)
   update-manager-core (1:22.04.19)
The following NEW packages will be installed:
   ubuntu-pro-client (31.1~22.04)
   ubuntu-pro-client-l10n (31.1~22.04)
The following packages will be upgraded:
   python3-update-manager (1:22.04.18 => 1:22.04.19)
   ubuntu-advantage-tools (28.1~22.04 => 31.1~22.04)
   update-manager (1:22.04.18 => 1:22.04.19)
   update-manager-core (1:22.04.18 => 1:22.04.19)
4 upgraded, 2 newly installed, 0 to remove and 351 not upgraded.
Need to get 835 kB of archives.
After this operation, 313 kB of additional disk space will be used.
Do you want to continue? [Y/n]
<---

I confirm then that launching and finishing update-manager without internet connection produces no error any more.

I got a Bionic ISO and run it in a virtual machine.

Enabled proposed and now upgrading update-manager forces the installation of a newer ubuntu-advantage-tools:

--->
$ sudo apt install update-manager --verbose-versions
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
   python3-update-manager (1:18.04.11.23)
   ubuntu-advantage-tools (31.1~18.04)
   ubuntu-pro-client (31.1~18.04)
   ubuntu-pro-client-l10n (31.1~18.04)
   update-manager-core (1:18.04.11.23)
The following NEW packages will be installed:
   ubuntu-pro-client (31.1~18.04)
   ubuntu-pro-client-l10n (31.1~18.04)
The following packages will be upgraded:
   python3-update-manager (1:18.04.11.13 => 1:18.04.11.23)
   ubuntu-advantage-tools (27.2.2~18.04.1 => 31.1~18.04)
   update-manager (1:18.04.11.13 => 1:18.04.11.23)
   update-manager-core (1:18.04.11.13 => 1:18.04.11.23)
4 upgraded, 2 newly installed, 0 to remove and 376 not upgraded.
Need to get 820 kB of archives.
After this operation, 1,570 kB disk space will be freed.
Do you want to continue? [Y/n]
<---

I confirm then that launching and finishing update-manager without internet connection produces no error any more.

I got a Xenial ISO and run it in a virtual machine.

Enabled proposed and now upgrading update-manager forces the installation of a newer ubuntu-advantage-tools:

--->
$ sudo apt install update-manager --verbose-versions
Reading package lists...
Building dependency tree...
Reading state information...
The following additional packages will be installed:
   distro-info (0.14ubuntu0.3)
   python3-distro-info (0.14ubuntu0.3)
   python3-update-manager (1:16.04.20)
   python3-yaml (3.11-3build1)
   ubuntu-advantage-tools (31.2~16.04)
   ubuntu-pro-client (31.2~16.04)
   ubuntu-pro-client-l10n (31.2~16.04)
   update-manager-core (1:16.04.20)
Suggested packages:
   shunit2
The following NEW packages will be installed:
   distro-info (0.14ubuntu0.3)
   python3-distro-info (0.14ubuntu0.3)
   python3-yaml (3.11-3build1)
   ubuntu-pro-client (31.2~16.04)
   ubuntu-pro-client-l10n (31.2~16.04)
The following packages will be upgraded:
   python3-update-manager (1:16.04.17 => 1:16.04.20)
   ubuntu-advantage-tools (10ubuntu0.16.04.1 => 31.2~16.04)
   update-manager (1:16.04.17 => 1:16.04.20)
   update-manager-core (1:16.04.17 => 1:16.04.20)
4 upgraded, 5 newly installed, 0 to remove and 210 not upgraded.
Inst python3-yaml (3.11-3build1 Ubuntu:16.04/xenial [amd64])
Inst distro-info (0.14ubuntu0.3 Ubuntu:16.04/xenial-proposed [amd64])
Inst ubuntu-advantage-tools [10ubuntu0.16.04.1] (31.2~16.04 Ubuntu:16.04/xenial-proposed [all]) []
Inst ubuntu-pro-client (31.2~16.04 Ubuntu:16.04/xenial-proposed [amd64])
Inst ubuntu-pro-client-l10n (31.2~16.04 Ubuntu:16.04/xenial-proposed [amd64])
Inst python3-update-manager [1:16.04.17] (1:16.04.20 Ubuntu:16.04/xenial-proposed [all]) [update-manager-core:amd64 ]
Inst update-manager-core [1:16.04.17] (1:16.04.20 Ubuntu:16.04/xenial-proposed [all]) [update-manager:amd64 ]
Inst update-manager [1:16.04.17] (1:16.04.20 Ubuntu:16.04/xenial-proposed [all]) []
Inst python3-distro-info (0.14ubuntu0.3 Ubuntu:16.04/xenial-proposed [all])
Conf python3-yaml (3.11-3build1 Ubuntu:16.04/xenial [amd64])
Conf distro-info (0.14ubuntu0.3 Ubuntu:16.04/xenial-proposed [amd64])
Conf ubuntu-pro-client (31.2~16.04 Ubuntu:16.04/xenial-proposed [amd64])
Conf ubuntu-advantage-tools (31.2~16.04 Ubuntu:16.04/xenial-proposed [all])
Conf ubuntu-pro-client-l10n (31.2~16.04 Ubuntu:16.04/xenial-proposed [amd64])
Conf python3-update-manager (1:16.04.20 Ubuntu:16.04/xenial-proposed [all])
Conf python3-distro-info (0.14ubuntu0.3 Ubuntu:16.04/xenial-proposed [all])
Conf update-manager-core (1:16.04.20 Ubuntu:16.04/xenial-proposed [all])
Conf update-manager (1:16.04.20 Ubuntu:16.04/xenial-proposed [all])
<---

I confirm then that launching and finishing update-manager without internet connection produces no error any more.

The verification of the Stable Release Update for update-manager has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.

This bug was fixed in the package update-manager - 1:22.04.19

---------------
update-manager (1:22.04.19) jammy; urgency=medium

  * d/control: Depend on ubuntu-advantage-tools >= 30~.
    Fixes edge case whereby the program crashes upon an incomplete
    response from ua security-status (LP: #2049785).
  * The New Release dialog should take precedence over a list of updates dialog
    if the latter would only show Ubuntu Pro updates in an unattached system.
    (LP: #2051115).

 -- Nathan Pratta Teodosio <email address hidden> Fri, 26 Jan 2024 15:10:31 +0100

This bug was fixed in the package update-manager - 1:20.04.10.20

---------------
update-manager (1:20.04.10.20) focal; urgency=medium

  * d/control: Depend on ubuntu-advantage-tools >= 30~.
    Fixes edge case whereby the program crashes upon an incomplete
    response from ua security-status (LP: #2049785).
  * The New Release dialog should take precedence over a list of updates dialog
    if the latter would only show Ubuntu Pro updates in an unattached system.
    (LP: #2051115).

update-manager (1:20.04.10.19) focal; urgency=medium

  * Add back removed widgets to UI file to fix crash when updating
    Upgrade Manager from inside itself (LP: #2045918).

 -- Nathan Pratta Teodosio <email address hidden> Mon, 22 Jan 2024 21:32:17 +0100

This bug was fixed in the package update-manager - 1:18.04.11.23

---------------
update-manager (1:18.04.11.23) bionic; urgency=medium

  * d/control: Depend on ubuntu-advantage-tools >= 30~.
    Fixes edge case whereby the program crashes upon an incomplete
    response from ua security-status (LP: #2049785).
  * The New Release dialog should take precedence over a list of updates dialog
    if the latter would only show Ubuntu Pro updates in an unattached system.
    (LP: #2051115).

update-manager (1:18.04.11.22) bionic; urgency=medium

  * Add back removed widgets to UI file to fix crash when updating
    Upgrade Manager from inside itself (LP: #2045918).

update-manager (1:18.04.11.21) bionic; urgency=medium

  * Fix incorrect available version for Ubuntu Pro updates in unattached case
    (LP: #2043425).

update-manager (1:18.04.11.20) bionic; urgency=medium

  * Ubuntu Pro (LP: #1990450):
    - fix another linter error which was created while fixing a missing
      space in the previous upload...

update-manager (1:18.04.11.19) bionic; urgency=medium

  * Ubuntu Pro (LP: #1990450):
    - Address linter errors that cause autopkgtest to fail.

update-manager (1:18.04.11.18) bionic; urgency=medium

  * Ubuntu Pro (LP: #1990450):
    - Show Ubuntu Pro packages, whether the system is attached to Ubuntu Pro
      or not.
    - Replace Install now button by Enable Ubuntu Pro button when only Ubuntu
      Pro packages are available and the machine is not attached.
    - Fix checkbox and expander widget from overlapping.
    - Add News pane in a expander.
    - Replace notebook with Description and Changes tabs by a pane.

update-manager (1:18.04.11.17) bionic; urgency=medium

  * Fix Ubuntu Pro updates checkbox and expander widget from overlapping
    (LP: #1990450)

update-manager (1:18.04.11.16) bionic; urgency=medium

  * Update of the parsing for pro client changes (lp: #1990450)

update-manager (1:18.04.11.15) bionic; urgency=medium

  * Show pending Ubuntu pro packages (LP: #1990450)

update-manager (1:18.04.11.14) bionic; urgency=medium

  * tests/test_meta_release_core.py: switch a test from using lucid to bionic
    as precise was removed from the archive. (LP: #1929865)

 -- Nathan Pratta Teodosio <email address hidden> Mon, 22 Jan 2024 21:36:03 +0100

This bug was fixed in the package update-manager - 1:16.04.20

---------------
update-manager (1:16.04.20) xenial; urgency=medium

  * d/control: Depend on ubuntu-advantage-tools >= 30~.
    Fixes edge case whereby the program crashes upon an incomplete
    response from ua security-status (LP: #2049785).
  * The New Release dialog should take precedence over a list of updates dialog
    if the latter would only show Ubuntu Pro updates in an unattached system.
    (LP: #2051115).

update-manager (1:16.04.19) xenial; urgency=medium

  * Add back removed widgets to UI file to fix crash when updating
    Upgrade Manager from inside itself (LP: #2045918).

update-manager (1:16.04.18) xenial; urgency=medium

  * Implement the new Ubuntu Pro design (lp: #1990450).
    - Fuse the description and changes tabs into a single view.
    - Add button to attach to enable Ubuntu Pro if it is the only action
      available.
    - Ubuntu base -> System components.
    - Fix Ubuntu Pro item not being a parent group of its corresponding
      packages.
    - Fix incorrect available version for Ubuntu Pro updates in unattached case
      (LP: #2043425).

 -- Nathan Pratta Teodosio <email address hidden> Mon, 22 Jan 2024 21:39:36 +0100

Hello errors.ubuntu.com, or anyone else affected,

Accepted update-manager into xenial-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/update-manager/1:16.04.21 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-xenial to verification-done-xenial. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-xenial. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

All autopkgtests for the newly accepted update-manager (1:16.04.21) for xenial have finished running.
The following regressions have been reported in tests triggered by the package:

update-notifier/blacklisted (arm64, armhf, ppc64el)

Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-migration/xenial/update_excuses.html#update-manager

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

Hello everyone who worked on this bug
It seems problems with the old Json parsing still happens even in ubuntu-advantage-tools > 30.
See https://bugs.launchpad.net/ubuntu/+source/ubuntu-advantage-tools/+bug/2058133.

I would again ask you to reconsider using the --format json, and moving to using the APIs (as it landed on Mantic + )

Using the supported and maintained APIs will indeed make update-manager more robust than relying on stdout output that ubuntu-advantage-tools does not maintain by default. We can, again, patch u-a-t for the new error we found, but using the APIs and dealing with exceptions would solve this particular problem IMHO.

I got a Xenial ISO and run it in a virtual machine.

Enabled proposed and now upgrading update-manager forces the installation of a newer ubuntu-advantage-tools:

--->
$ sudo apt install update-manager --verbose-versions
...
The following packages will be upgraded:
   python3-update-manager (1:16.04.17 => 1:16.04.21)
   ubuntu-advantage-tools (10ubuntu0.16.04.1 => 31.2.2~16.04)
   update-manager (1:16.04.17 => 1:16.04.21)
   update-manager-core (1:16.04.17 => 1:16.04.21)
<---

I confirm then that launching and finishing update-manager without internet connection produces no error any more.