Hi Fabio,
I think the endsystem is giving a SOAP:FAULT response. Try to add this parameter XMBWS.NoSOAPIgnoreStatusCode with value true (Note 1055678:XI 3.0 soap receiver in nosoap mode with HTTP status code). You will need to work with Do not Use SOAP Envelope checkbox marked and to create manually the SOAP envelope at mapping level (SOAP faults with "Do not use SOAP envelope" | SCN)
Regards.