Root and every other user worked fine for me to break out.
You might want to double check if the command su even is accessable, or sth. like that ... did you double check your script given by Falkos How2?
P.S. Btw. if you read that Falko ... I still don't understand why you didn't include that into the ISPconfig installation script ... ? A quick check if it's Debian Linux, or Ubuntu ... and quick messagedialog if you want it or not ... (because I think most of your ISPconfig testers are running ether Ubuntu or Debian)