Although I couldn't get the system past the disk check, and it seems to have a few unhappy blocks, the server has booted again.

For whatever reason the disk partition won't scan on boot. Maybe it's because the partition is mounted as the /var folder. It might get mounted early enough in the process that it can't be checked at boot, maybe?

I have peeked at the contents, and did find about 800GB of stuff that can probably be removed because it's backed up elsewhere. Probably an old backup folder that is now on the proper archive disk that I forgot to remove when I added more storage, like forever ago.

When I have a little time I'll boot the disk with a live USB thumb drive and run the disk check. Until then, it's running again.