Hello Jan,
You might try to "watch" on the wire what's happening with a tool like TCP Mon: http://code.google.com/p/tcpmon/
Place it in between PI and Sharepoint as "man in the middle" to see what's exactly going
to Sharepoint as a request and coming back as response.
You could do then the same with your SOAP UI call and compare the differences, might give some hints.
With best regards
Sebastian