Vincent,
Yes, the error message you are getting is unfortunate and not very descriptive. You should only get this if the authentication fails and the class is unable to parse the returned error for some reason. I recommend downloading and installing Fiddler2 (http://www.fiddler2.com) or a similar application to have a look at the actual data being sent to the server and especially coming back when you call UpdateCredentials(). Please post the content of the result you are getting (or the HTTP result code coming from the server), that should help figure out what's going on.
//TB