Comment 4 for bug 1534090

The only reference I found to get_tasks_recursive seems far away in cgmanager (http://changelogs.ubuntu.com/changelogs/pool/main/c/cgmanager/cgmanager_0.32-4ubuntu1.1/changelog and https://linuxcontainers.org/it/cgmanager/manpages/man1/cgm.1.html)

Yet I found that my Xenial Test system didn't show the issue, while Michael before mentioned it did for him.
I realized that this Xenial System had no cgmanager installed.

So I installed full lxd (which is what usually brings cgmanager to your system) and e voila I was able to reproduce the issue on Xenial as well now.

The output is slightly different (more and repeated on Xenial)
Trusty
call to get_tasks_recursive failed: Invalid arguments received in reply

Xenial
call to remove-on-empty (freezer:0) failed: invalid request
call to get_tasks_recursive failed: Invalid arguments received in reply
call to get_tasks_recursive failed: Invalid arguments received in reply

I removed lxd and all its dependencies again and only installed cgmanager on the Xenial system. This pulls in libcgmanager0 and libnih-dbus1 as well, but nothing else.
With those installed the issue does not yet appear, so it seems to need more of these packages to actually trigger.
I tried to separate the lxd dependencies and found that libpam-cgm, likely to hook from pam (which is what su uses) into cgmanager.