I have ported web application to WAS WebServices FP). Application use MTOM. In WAS, when MTOM is enabled, the binary data that represents the attachment is included as a MIME attachment to the SOAP message, but when message doesn’t have attachment there isn’t multipart:
Message without attachments on WAS:
{code}HTTP/1.1 200 OK
SOAPAction: ""
Content-Type: application/xop+xml;type="text/xml"
… {code}
In .NET client that causes an error: The content type application/xop+xml;type="text/xml" of the response message does not match the content type of the binding (multipart/related; type="application/xop+xml").

The same message without attachments on original server :
{code}HTTP/1.1 200 OK
SOAPAction: ""
Content-Type: Multipart/Related; boundary="----=_Part_4626_1190020846.1250503177036"; type="application/xop+xml"; start-info="text/xml"
Content-Type: application/xop+xml; type="text/xml"; charset=utf-8

Can it be a problem with the configuration and installation of WAS?