I think i found a possible but simple solution.
If you replace the line
$rcmail_config['default_host'] = 'ssl://localhost';
in the RoundCube configuration file with:
$rcmail_config['default_host'] = str_replace("www.","",$_SERVER['SERVER_NAME']);
Then in your browser, go to http://www.yourdomain.tld/webmail
As you can see the server field at the RoundCube login screen is hidden and yourdomain.tld will be used as the server name. The result is a succesful login from every ISPConfig server in my network.
The problem is that it only works when you type http://www.yourdomain.tld/webmail
in your browser. It does not work with names other the "www". Maybe i'm on the right track and someone can make it better then this.