"Oops" ... lists down for a bit. Drats ... and hopefully all better by the time this mail makes it to list.
So ... all was fine and dandy until ... managed to have a host booboo, basically locked up solid, and I power cycled it (this was physical host upon which the VM was running). Shouldn't be any biggie ... after that all up and fine again ... except the Mailman 3 lists.
So, ... isolating and working towards fixing that. I'm guestimating maybe issue with some kind of lock file or the like that didn't get cleaned up upon (re)boot, or possibly, since I perhaps didn't do as many reboots as I ought to have made sure it would "always" come up clean, perhaps there was some misconfiguration or the like that somehow snuck in, that would be effectively a latent defect/issue, and that wouldn't show until a reboot or attempted restart of the relevant service(s).
At this point still troubleshooting to narrow down the issue.
As far as I can tell so far, looks like web server hits some kind of issue and generally gives or passes along a 500 response, that then ends up with the relevant web page(s) failing. When I did deeper into the backend service(s) ... notably postorius, looks like it gets requests okay and responds to them okay ... but something goes wrong somewhere between there and web server properly getting and passing that along to client browser. E.g.: https://lists.balug.org/mailman3/postorius/lists/ I see in postorius bit returning (from strace(1)) including: {"display_name": "BALUG-Admin", "fqdn_listname": "balug-admin@lists.balug.org", {"display_name": "BALUG-Announce", "fqdn_listname": "balug-announce@lists.balug.org", {"display_name": "BALUG-Talk", "fqdn_listname": "balug-talk@lists.balug.org", {"display_name": "BALUG-Test", "fqdn_listname": "balug-test@lists.balug.org", But somehow that doesn't make it to the web page the server serves ... so I'm guestimating there's some issue somewhere between postorius and Apache.