IDOC Mapping ; Error CX_XMS_SYSERR_MAPPING

After we transfered an IDoc 1:1, w/o mapping, successfully from one to another SAP system, we added a mapping like follows:

1. created a message mapping per drag&drop from imported objects - IDocs, using a 1:1 mapping (no field changes, just for testing purposes)

2. created an interface mapping using that message mapping.

3. after activating in repository, we added that interface mapping in interface determination in configured inbound-interfaces.

After starting a test data transfer we received the error message

"CX_XMS_SYSERR_MAPPING Programm: CL_XMS_MAIN===================CP, Include CL_XMS_MAIN===================CM00A, line: 535"

And an odd error id with strange crosses in the overview.

Did we forget something? Any hint is welcome.