Odhiambo Washington via Mailman-users writes:
Your first and greatest mistake is that you installed Mailman using OS packages and that is NOT supported on this list.
Of course we support people who are using OS packages. You don't have to if you don't want to, but please don't suggest they should go away. And your advice to wipe and reinstall from PyPI is often appropriate, but that is not so here because they have a working site already. Only one new domain is failing.
We do recommend *against* using using OS packages, because we can't support them very well, and the Debian family tends to let the packages get quite old before updating to recent Mailman releases. So for many kinds of problems, the first-line support for OS package users *should* be the OS distribution. It's easy enough to tell them that when it's so.
In this case, Flo says 3.3.8 is installed, which is almost 3 years old now. It's quite possible there's a Mailman bug in that version we have since fixed. Unfortunately, I don't have a clue. Maybe Mark does.
You can remedy that by wiping away the packages and installing MM3 by following the guide below:
@Flo: It is possible to do this safely and preserve your existing sites and lists. If we can't help you with your existing installation, we can guide you through this process. The basic idea is that because almost all the interesting data is in your database or /etc/mailman3, you can just plug in the new Mailman system, point things at the right places (probably the file system layout will change so you'll need to change some things in mailman.cfg, settings.py, and some related systems like logrotate and systemd).
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan