Assuming you have access to your configuration files, try editing login/index.php (in my setup, this is at /var/www/html/login/index.php).
Find the line that says
$authplugin->loginpage_hook();// $authplugin->loginpage_hook();Moodle should then fall back to its default authentication method, i.e. email + password.
Let us know whether this works, or whether you've found another solution.
~ William
Source: Suggested here by Daniel Thies