peter@pi4cc.nl writes:
I'm running Debian 12 with the latest version and installed mailman3 via apt install-mailman3-full.
Assuming you have done no configuration "by hand", this is almost certainly a Debian issue. You should report it to them.
When I first access the website domain.nl/mailman3, it's redirected to domain.nl/mailman/postorius/lists/, resulting in a 404 error
The initial redirect is done by your webserver, not by Mailman. I have no good idea why the result changes if you do it twice.[1] In Apache, this is likely in a file in /etc/apache2/sites-available/. In nginx, similarly /etc/nginx/sites-available/. The file's name is likely to be either something like "debian-default.conf" or "mailman3.conf" for either of those popular webservers.
Footnotes: [1] The only thing I can think of is that in one case you accessed domain.nl/mailman3, and in the other domain.nl/mailman3/ (difference is the trailing "/"), and there are separate redirects for the two URLs for some reason.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan