I'm no specialist, but I had some problems myself during the configuration.
1. Did you download the manual? Everything is described into details!
2. During the installation of ISPconfig3, did you enable SSL? If, try "https"
3. Try to open all ports, choose DMZ --> yourispserver ip-number, loose all defense. Turn it back on when you find the error.
4. Was it an clean installation or was it an installation on a server you have been using for something else?
5. It's not much information you provide, but is it Ubuntu Server or Ubuntu Desktop?
Please provide some more information regarding your system. I have just succeded with installing my ISPserver and I can tell you, most of the errors were "error 40!" 40cm behind the screen