If you see messages like this in the log of the MDrivenServer:

Ambiguous values for <class>.<association> for id <xxxxx>. The following objects (ids) all refer to the same target: yyyyy, zzzzz

For a single link: somehow, two rows in the database have the same ID pointing to the same object, which is wrong for a single link!

The easiest way to check what is wrong is to use the Validate data in the database.