I had a similar problem and struggling, googling for 1 day. But it was a simple mistake that instead of:
{http://ws.somecompany.com/services}ValidateUser
It should be
{http://ws.somecompany.com/services/}ValidateUser
This version of XHTML provides a definition of strictly conforming XHTML 1.0 documents, which are restricted to elements and attributes from the XML and XHTML 1.0 namespaces. See Section 3.1.2 for information on using XHTML with other namespaces, for instance, to include metadata expressed in RDF within XHTML documents.