When we moved to the new server with iis7 dotnetopenauth quit working with yahoo and openid.

The solution to the problem was to set the app pool identity to "load user profile" whatever that means. But it fixed it after finally stumbling on to the right thread.

http://stackoverflow.com/questions/1983425/policyexception-when-using-dotnetopenauth-library/1987800#1987800