Hi Armin
First of all, did you define a fault message type for both sender and receiver service interface, and linked them via a fault mapping in an Operation Mapping?
If you already did that, can you try adding the TraceHTTP parameter in your SOAP receiver channel as described in the blog below.
HTTP tracing in SOAP receiver adapters with TraceHTTP module parameter
Once you have done that, can you share screenshot of the HTTP response in the Log Viewer? In particular, I would like to see the actual SOAP response from the web service when the error occurs. This is because section 7 of SAP Note 856597 - SOAP adapter FAQ mentions that only if the fault detail element is available in the response, then a fault payload will be returned. If it is not available, then a system error is generated instead.
Regards
Eng Swee