I didn’t help but I managed to get Roon to see my files again.
In Settings > Storage I added a folder and then created a new folder. I then put a bunch of music files into that folder and finally forced a rescan of that folder. Three files were imported. Then I forced a rescan of the other two folders (which did nothing in the past) but now my files are found again.
Looks to me like the database was somewhat broken and adding a new entry fixed the previous ones. That shouldn’t happen!