try replace in includes/sessions.php
WHERE ".get_user_table_field("", "user_name")." = '$user_name' AND ".get_user_table_field("", "user_level")." <> ".USER_AWAITING;
with this:
WHERE (".get_user_table_field("", "user_name")." = '$user_name' OR ".get_user_table_field("", "user_id")." = '$user_name') AND ".get_user_table_field("", "user_level")." <> ".USER_AWAITING;