Mine is doing the same thing. It happened yesterday morning, when some packages were upgraded by the system. I'm glad it's not just me this has happened to.
It looks like someone pushed an apache2 package which broke things on the 28th September, and another has been pushed yesterday, but it's still broken. From my logs, these packages got updated the first time:
Start-Date: 2021-09-29 06:38:15 Commandline: /usr/bin/unattended-upgrade Upgrade: libapache2-mod-proxy-uwsgi:amd64 (2.4.41-4ubuntu3.4, 2.4.41-4ubuntu3.5), apache2-data:amd64 (2.4.41-4ubuntu3.4, 2.4.41-4ubuntu3.5), apache2-bin:amd64 (2.4.41-4ubuntu3.4, 2.4.41-4ubuntu3.5), apache2:amd64 (2.4.41-4ubuntu3.4, 2.4.41-4ubuntu3.5), apache2-utils:amd64 (2.4.41-4ubuntu3.4, 2.4.41-4ubuntu3.5) End-Date: 2021-09-29 06:38:52
and then the next day:
Start-Date: 2021-09-30 06:33:46 Commandline: /usr/bin/unattended-upgrade Upgrade: libapache2-mod-proxy-uwsgi:amd64 (2.4.41-4ubuntu3.5, 2.4.41-4ubuntu3.6), apache2-data:amd64 (2.4.41-4ubuntu3.5, 2.4.41-4ubuntu3.6), apache2-bin:amd64 (2.4.41-4ubuntu3.5, 2.4.41-4ubuntu3.6), apache2:amd64 (2.4.41-4ubuntu3.5, 2.4.41-4ubuntu3.6), apache2-utils:amd64 (2.4.41-4ubuntu3.5, 2.4.41-4ubuntu3.6) End-Date: 2021-09-30 06:34:32
So clearly something's gone wrong with Ubuntu's apache2 packages, and it's broken something quite badly for Django and/or Postorious. But I don't know enough about how Django or WSGI work to be able to debug this.
When I look at the mailman-web-logs, the urls that postorious puts into the page don't honour the POSTORIOUS_TEMPLATE_BASE_URL variable in the mailman-web.py file any more.
Tim