I knew the server was running out of space. It was over 90% full, so I knew I'd have to deal with it at some point.
When Buster told me the website wasn't working, I logged in to see what was wrong. I saw there were updates and thought I might as well do those first. When the updates failed, the error message told me the drive was full!
While it couldn't generate the Forum, it did spit up an error message. Such is the power of Linux! I doubt that would happen if a Windows server were full, but I'm guessing. Anybody care to comment?
Anyway, I doubled the size of the drive and the problems disappeared. Easy fix. And it didn't even cost more. Turns out I could have upped the server size long ago, but didn't see the option then. Doh!
The moral of the story? Don't kick potential problems down the road like I did. Be proactive.