Hi Ashwin,
thanks for your reply.
I tested it again with my ContractVersionSet and when I return the empty er_entity it is unbound by the GW framework and the /IWBEP/CX_MGW_BUSI_EXCEPTION is raised with the "Resource not found" message which seems to be by design.
When this GET request is contained in a $batch call, then the return is like
"In the context of Data Services an unknown internal server error occurred".
Ok, this explains the difference: If any operation in a batch fails, then the entire batch fails and only this "internal server error" is returned instead of the concrete "Resource not found error".
I think this is because in a batch multiple operations could fail, so which error should the GW framework report then?
Therefore it just reports that there has been some "internal error".
Everything clear now.
Thanks for your support!