Hi Dimitri,
first of all please excuse my late reply - I was happy to enjoy a 3 week holiday.
In the meantime SAP replied to my OSS call and pointed to Note 2039583 for solution.
I've patched my B2B SP3 to the latest patch level and the problem was solved.
So thanks for your help - if you still want to have details just give me a note.
Kind regards
Markus