Fixed in version 3.3.10 commit 168e6f55: https://gitlab.com/mailman/mailman/-/commit/168e6f5557c630a733dfc3b51dac47cb... Michael Paoli wrote:

After unclean shutdown of mailman3, even when (re)started with start --force mailman3 may fail to remove the lock files and thus fail to (re)start. Details on (I submitted as): https://gitlab.com/mailman/mailman/-/issues/1174 I'm definitely not the first person to have noticed this issue. Hopefully they'll get around to fixing it (it's been over 9 years ... but they may have lacked sufficient relevant detail in the earlier reports). Manual workaround: remove the lock files in /var/lib/mailman3/locks/ after checking to ensure that none of them reference and/or are used by any PID(s) still relevant to mailman3