Login broken on update

Upgrading wp-login from 1.7 to 2.2.2 completely breaks login… No error messages, it just reloads the login page when trying to log in with any account. I do suddenly get an ‘unsecure HTTPS connection’ message. I fixed it by putting the old 1.7 files back.

Oddly enough, this only happens on the live site, not on my VM dev version. But these sites should be identical… Any clue what’s happening here?

http://wordpress.org/plugins/rename-wp-login/