I am getting a File Error that references a .MST file when I attempt to open a ChemFinder database.
That kind of thing might happen when the ID's in the mdb file get out of synch with the mst file. It might be that there is a MolID in the mdb, but that exceeds the number of entries in the mst.
Two things you might try:
1) Use File Convert and convert the database to the same or a different version. This compacts the structure file and rebuilds the index.
(2) Use the Structures button on the Table tab, and reconnect to the mst file. This gives some info about the number of ID's in each part and offers to fill in missing ones.