Got this message after updating the newest Patch release on ubuntu 12.04. apache did not start up. Same for port 8081. Port was free. I tested this with firing up a listening netcat on 8080. The Listen Statement was only in /etc/apache2/sites-available/ispconfig.vhost. After moving the Listen to /etc/apache2/ports.conf it works again. So somehow the Listen in ispconfig.vhost has to be used twice. But I cant find a second reference.