package mailman 1:2.1.14-3ubuntu0.1 failed to install/upgrade: subprocess new pre-installation script returned error exit status 1

Status changed to 'Confirmed' because the bug affects multiple users.

The data here clearly shows the issue for the reason.
From your log:
 The directory /var/lib/mailman/qfiles contains files. It needs to be
 empty for the upgrade to work properly. You can try to handle them by:
  - Stop new messages from coming in (at the MTA level).
  - Start a mailman queue runner: /etc/init.d/mailman start
  - Let it run until all messages are handled.
    If they don't all get handled in a timely manner, look at the logs
    to try to understand why and solve the cause.
  - Stop it: /etc/init.d/mailman stop
  - Retry the upgrade.
  - Let messages come in again.
 You can also decide to simply remove the files, which will make Mailman
 forget about (and lose) the corresponding emails.
...
Package configuration
 Configuring mailman
 Old queue files present
 abort installation

Mailman needs the "active incoming queue" to be empty on upgrade.
That is a local configuration issue and not a bug in the package.
Gladly the fail even shows how to resolve it.

Thank you for your report.

This looks like a local configuration problem, rather than a bug in Ubuntu.

You can find pointers to get help for this sort of problem here: http://www.ubuntu.com/support/community

Since we use this bug tracker to track bugs in Ubuntu, rather than configuration problems, I'm marking this bug as Invalid. This helps us to focus on fixing bugs in Ubuntu.

If you believe that this is really a bug, then you may find it helpful to read "How to report bugs effectively" http://www.chiark.greenend.org.uk/~sgtatham/bugs.html. We'd be grateful if you would then provide a more complete description of the problem, explain why you believe this is a bug in Ubuntu rather than a problem specific to your system, and then change the bug status back to New.

[Expired for mailman (Ubuntu) because there has been no activity for 60 days.]