On the ServerFault login page, the OpenID and other parts of the form are hidden. Only Google, FB and Email/passwd are shown.
I hacked page's DOM with Chrome Developer Tools, got the fields to display, and got the login to work. (There was also a javascript test for user-name, but then that glitched or something on the second try.)
Anyhow, thought you ought to know. (Also, thanks for the interesting puzzle!)