The problem has now been solved. The solution was to use http instead of uwsgi as protocol. Also, communication is running through http instead of a socket. I can't tell if http protocol would also work through a socket, but uwsgi wasn't working through either. Maybe the uwsgi.ini should contain a hint to that possibility in the future. Also, the apache config that is created during installation seems to be incomplete, as it only contains mailman3 but not hyperkitty, django and admin, plus mailman3 also isn't working and had to be replaced by postorius. Not sure though if that was only a problem for me or if that should be added.
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