Well, I got one step further by monitoring the traffic with Eclipse's TCP/IP Monitor. What I found out is that in case of the stand-alone Java sample client the SOAP request has just an envelope and a body - which works fine - but in case of running the same code in the app server an empty header element is added into the envelope. This seems to break it as the remote web service runs into an error then.
So, my question is, how can I avoid that a header element is automatically added? Any thoughts?