Roon ignores my music

Roon Core Machine

QNAP TVS-682
Intel Core i3-7100
8 GB RAM

Networking Gear & Setup Details

AVM FRITZ!Box 6591 Cable
Ethernet only

Connected Audio Devices

3.5 mm audio jack on Roon Client

Number of Tracks in Library

approx. 16,000 tracks

Description of Issue

Everything was working until Roon suddenly didn’t showed my local music anymore. Roon Core has read and write access to my audio files and they can be seen in the Roon client Settings > Storage > Add folder dialog but they are not shown in Roon itself anymore. In Settings > Library > Clean up library it showed that I could delete 14675 (or so) files that aren’t there anymore (well, the files are still there). I did so. I removed the storage path. Restarted Core and Client. Added the correct storage path again but Roon doesn’t want to index the files.