Many thanks! It worked!
Lesson learned: whenever it’s necessary to recreate the library database (~/.config/darktable/library.db), the cache needs also to be recreated.
I wonder if one of the latest commits that changed the database could have missed cleaning the cache. I’m saying this because I don’t remember having manually reset the library database myself lately, though I’m not 100% sure about that.
EDIT: Oh, and welcome to this community. And thanks for helping develop this great software!