This is a late edit of my previous post:
WOW! Don't start over! But I also suspect a corrupted database.
So, simply exit QMC2, then delete all the .db (file type 'database') files from your profile (.QMC2). Be careful not to delete the .ini files. Then rerun Qmc2 and wait while the db files are recreated. Then 'check roms'.
I do believe this will fix your problem.
Using the 'exit - stop all active processing' button often corrupts the database. NEVER use it! If it were my product, I would remove it! If it is pressed during startup, it will result in truncated or mismatched db files.
I call it 'the corrupt the database button'.