I just had that problem a few days ago.
I'm working with VaSt 6.0.2 and called a websphere hosted webservice.
Set a breakpoint at: "SstWSClientEngine invokeHandlers: anSstWSMessageContext"
Don't let results be called on anSstWSMessageContext, that throws the exception...
Inspect anSstWSMessageContext after invoking. It then contains the whole information of your request and client-environment including the response content.
My fault was that the answer, I got from the server, wasn't a SOAP-XML. It was just a HTML-message containing 403 (not allowed) error send by the server.
Write an exception handler for this und catch that soap-error passing the context...
Sebastian (awaiting 8 beta)