Hi Mark,
I think that I found a hint as to the cause of my problems.
I changed the apache2 LogLevel to debug and the following was then generated in /var/log/apache2/ssl_error_log:
[Fri Feb 11 15:40:18.325675 2022] [core:debug] [pid 3982011] core.c(5280): (95)Operation not supported: [remote /var/lib/mailman3/web/static/uwsgi.sock:12150] AH00139: apr_socket_opt_set(APR_TCP_NODELAY)
Apparently, there appears to be a problem with uwsgi.sock.
My uwsgi.ini file has the following:
uwsgi-socket = /var/lib/mailman3/web/static/uwsgi.sock
Also, my apache.conf for mailman3 has the followig:
ProxyPass /mailman3 unix:/var/lib/mailman3/web/static/uwsgi.sock|uwsgi://localhost
So do you see a problem in my configuration files that would cause the Operation Not Supported error with the uwsgi.sock file.
Thanks,
Gordon