Data corruption combined with an operating system upgrade created an unstable database. I tried rolling back to older database snapshots, but none were working until I got all the way back to March 2024. Once I got that working, I tried applying the now stable templates to the latest backup. It failed. However, the previous one worked!

Drupal 7 goes End Of Life January 2025, so a software update is inevitable. Hopefully it won't suck. Wish me luck...