Hi,Martin
we were facing the same problem before.it is just the verison match problem.
MDM Java API and MDM Connector are two components for mdm, they are needed to be depolyed on the portal's java Application Server.
for your mdm catalog sp15,it is tested with the mdm 7.1.10.204, it means that you need to use the MDM 7.1 SP10 Patch204 or above Patch,also read the MDM's release note.the java API, MDM Connector version must be the same with the MDM server.
by the way,take the portal version and SRM version into consideration,check the note to match all the versions.
best regards
Stephen