On SailfishOS it is located here:

/home/nemo/.config/harbour-quasarmx-full/quasarmx.sqlite

Just delete it and a new database will be created at the next start of QuasarMX.

The exclamation marks in front of the items are caused by QuasarMX not finding the required files. This happens on SailfishOS when the ID used for the SDcard mountpoint changes.
Can you still re-scan your collection?