Okay, but I've found an bug with the api (https://steamcommunity.com/discussions/forum/7/1696044439570067964/) but that's nothing to do with this addon.
Steam changed from http to https request. Open the file 'src\addons\BlackTea\SteamAuth\ConnectedAccount\Service\Steam.php' and change Line 113 from:
$ptn = "/^http:\/\/steamcommunity\.com\/openid\/id\/(7[0-9]{15,25}+)$/"; to:
$ptn = "/^https:\/\/steamcommunity\.com\/openid\/id\/(7[0-9]{15,25}+)$/";