Well, I rebuilt the database from the scratch and currently it works, even having a filesize of 2.01 GB.
I'll have an eye on it and will give feedback in case I find that issue reproducable.

May I add a point to this discussion? I discovered the Mediamonkey database grew up to above 2.1 GB in size on my harddisk. Even after a reorganization of DB it still is 2.1 GB. When I try to add more files it refuses to take more elements into the DB. The music list contains about 830 k entries. I ...