Plex is having issues on the server. I'm trying to figure out why, but so far troubleshooting has not yielded any results. My current fears are that the Plex database is having issues or there's a hardware issue of some sort (the server has been on basically 24/7 for 5 years after all.
Consider Plex to be working intermittently (regardless of the status below) while I try to figure it out. But please let me know directly if you're having an issue until I figure it out.
If any of the bubbles on the left below are Red, that service is down and not available. I probably already know about it, and am probably working on it. If it's green below and not working, let me know.