Please try to reach the controlpanel with your IP address instead of the domain. Replace the IP in the exampls with your server IP.
https://192.168.0.1:81
or if you chse http in the installer:
http://192.168.0.1:81
If this wont work, please execute:
/etc/init.d/ispconfig_server start
and try again. If this wont help, please post the output of the command:
netstat -tap
here.