Hi Ingo,
do u have try to reset the MDM Java API?
You can access it via the following URL:
http://<host>:<port>/mdm/apireset?MDS=<MDS host>[:<MDS port>]
Additional to this, if the MDM API reset it not works please restart the MDM connector (removing invalid connections from the connection pool)
i hope this helps.
br,
fabian