Comment 3 for bug 1446941

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.