Flo via Mailman-users writes:
In /usr/share/mailman3-web/settings.py there is:
INSTALLED_APPS = ( ... 'django.contrib.humanize', ... )
Right, that's why Python is looking for it. Since it's not being found, I guess there's a version skew between Mailman and django.contrib. The first thing to do is to make sure all your packages are up to date. Check if the django package (or maybe django-contrib if it's a separate package) is pinned to a particular old version, and consider unpinning and upgrading it. If they are up to date, and you're skilled with Debian, you may consider adding more recent package sources (testing or experimental) and install Django from those sources.
If that's not appealing I would strongly discourage trying to add the humanize package from Django upstream. That is a real can of worms. If you wish to continue using the Debian packages, you will need to get help from the Debian maintainers. I wouldn't be surprised if they already are planning to do something about this issue.
The alternative would be to uninstall the Debian package and install the most recent Mailman packages from PyPI in a Python virtual environment /opt. This probably requires installing the Debian python3-venv package. This is not terribly difficult, the instructions at docs.mailman3.org/en/latest are good, and you're welcome to ask for help on this list. We recommend this because you will get better support that way.
And POSTORIUS_TEMPLATE_BASE_URL = 'http://localhost:8001' I think this has also something to do with it.
That has nothing to do with it, they are quite unrelated.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan