Steam recently changed the community urls returned to use https:// instead of http://, which broke authentication.
Change Http:// to Https:// in library/Steam/ControllerPublic/Register.php. I have added the change below on line 657:
Code:
// Validate wheather it's true and if we have a good ID
preg_match("#^https://steamcommunity.com/openid/id/([0-9]{17,25})#", $_GET['openid_claimed_id'], $matches);