hello Tim and Juilius,
the crash of SAP GUI once the SNC has been enabled can be only be overcame giving SAP_ALL to TMSADM on client 000 of the destination systems.
This way it works however it has the very bad side-effect that all the transport will be transported with the users TMSADM so we decided to dismiss this scenario, disable SNC for STMS and rebuild the RFC in every single system involved.