The Apache Error Log says:
authz_core:error] [pid 90530] [client 50.200.240.158:50063] AH01630: client denied by server configuration: /var/lib/mailman3/web/static/index.php
So seems to be a permissions issue, but www-data owns that folder and subdirectories.
For the django settings (mailman-web.py) it is set to allow all hosts:
ALLOWED_HOSTS = [ #"localhost", # Archiving API from Mailman, keep it. # "lists.your-domain.org", # Add here all production URLs you may have. '*' ]
No errors in the mailma3-web.log
On a whim I tried to load the SSL certificates for lists.example.com to see if maybe it was causing the issue. Now when navigating to lists.example.com we got a ProxyError:
Proxy Error The proxy server received an invalid response from an upstream server. The proxy server could not handle the request
Reason: DNS lookup failure for: localhostpostorius