Found a bug. Custumer has a Flickr account and want a website redirect. They use @ in their links. ISPconfig does not accept that. Example: https://www.flickr.com/photos/139077275@N07/albums Solution is to manually edit the link in Apache Vhost.
Actually, we do not allow @'s in redirect URL's in ISPConfig. If you want to allow that on your server, alter the regex. Or put the redirect in a .htaccess file of the site or the apache/nginx directives field instead.