Hi Timm,
Could you post what error you received in the backend - Use transaction /IWBEP/ERROR_LOG. It will be more clear then. Also if you want to check in the Gateway, please put a breakpoint in class /IWFND/CL_SODATA_PROCESSOR, method /IWCOR/IF_DS_PROC_ENTITY_SET~READ and debug.
Best regards,
Aakash