So I went digging, found the code resulting in this error message and found out the key is forbidden to be generated for Admin. I have no idea why, it probably boils down into wordpress intenstines somewhere but it has nothing to do with this plugin.
What I would suggest is parsing out the received array from:
$key = get_password_reset_key( $user_data );
on line 354 of file smfcp-functions.php and showing the error saying the actions is forbidden for admin account or so.
To be fair, it returns the key just fine if the actions is possible and returns an array with error if it is not, so simple is_array($key) would probably work.