Hi - sure there is no share/folder on your NAS hinting to some kind of Roon backup? If that wasn’t working Roon would’ve indicated a problem while you still had a database.
You are not alone. I’ve had the same happen twice, and it presents as a mystery to support (“the database is designed to prevent this from happening”). Eventually, they’ll tell you to rename your current database folder and let Roon create a new database. You’ll lose your playlists, etc., but not your music. And, you’ll have to do it all over again in a couple of months.