If its a new install, then I would change the machine key, clear the browser cookies before you login, and that should solve it. I actually added error handling there for an error I got with this during testing, but I was getting a cryptography exception, I'm kind of surprised it would throw a more general httpexception there. Is this after installing the new security update patches form Microsoft? I wonder if the update changed the error. I will add logic to catch this httpexception.