[MM3-users] Again, after systemupgrade the log file in /var/log/mailman was owned by list:list instead of mailman:mailman and starting failed for this reason [support request]

Wikinaut writes:

For some unknown reason, an empty file /var/og/mailman/mailman.log was apparently owned by list:list. I deleted that file and mailman3 core could now be started. The start problem i solved.

I'm guessing you have a Debian or Ubuntu system, and there's some remnant of a Mailman installation (can't guess if it's 2 or 3). When you upgrade the package configuration script did it.