Still trying to figure out exactly what conditions led to this issue, but it was a doozy.
I'm on 11.5.3 and I back up a small number of Mac Pros with multiple internal drives. Dipped into a backup recently looking for a file that became corrupted (unrelated to Retrospect). To my surprise, one of the drives on one of the machines had no files newer than 3 weeks ago. There had been plenty of "successful" backups of that drive in the intervening 3 weeks, and other drives on that machine did have new files backed up (I think?). But not that one.
Contacted support; after some troubleshooting, they noticed that the Instant Scan file on the client machine hadn't been updated for 3 weeks. Disabling Instant Scan fixed my issue and returned backup processes to normal.
It did not, of course, restore my corrupted file, which cost significantly more than a Retrospect license to replace.
Moral of the story: be very, very careful using Instant Scan, as it seems to be able to fail to back up files for weeks without knowing that it's failing. Also note that AFAIK, the only way to disable it is client-by-client, holding down Command before clicking on the icon for the client in System Preferences.