Looks like we have another issue using social network account for logging in. In this case is Facebook. I logged out, cleanup my history. Log back with Facebook account then open UKAPS. First attempt login does not work. It returns to login. Hit login with Facebook again and login successfully.
OS: OS X El Capitan
Browser: Safari 9.1.2
OS: OS X El Capitan
Browser: Safari 9.1.2