I believe that we sorted this out over email. If you look at the details of the error message, you should see more information. What's happening is the webservice is returning an error page in HTML rather than the expected XML data.
In this case, it indicated a 500.21 "server application unavailable" error, which probably has something to do with the IIS application pool servicing the UAAService application. It is either running in the default app pool with ASP .NET 1.1 applications, or the app pool is configured to use "integrated pipeline" mode when it should be using "classic" mode.