Problem solved.
I noticed that the entire bwdata directory had switched user:group to nobody:nogroup. I wondered if that had to do with it. I looked at uid.env, and sure enough user and group were set to 65534 in there.
I changed them back to bitwarden UID and GID, rebuilt and restarted, and now it works fine.
I have no idea when uid.env changed to 65534, I’ve never paid any attention to the user:group of the bwdata directory. I’ve been running this installation since 2021, and it was a migrated database (and bwdata directory) from another server from 2019, so many years of flawless running and updates.