Search in includes/page_header.php for
"url_register" => (!empty($url_register)) ? $site_sess->url($url_register) : $site_sess->url(ROOT_PATH."register.php"),
and replace it with
"url_register" => (!empty($url_register)) ? $site_sess->url($url_register) : $site_sess->url("https://www.yourdomain.com/register.php"),
Jan