I've tested the RoundCube package on two different Debian Lenny servers and in both situations the RC package works fine.
So don't think there is anything wrong with the roundcube package.
It also might be that you've a cache problem, maybe restarting ispconfig by executing the command /etc/init.d/ispconfig_server restart
(I remember that this helped in the passed with previous versions of RoundCube and also phpMyAdmin.
It is very important that your server offers all the system requirements for RoundCube.
You can check if that is the case by enabling the RoundCube installer tool.
The installer tool can be found at URL http://www.example.tld:81/roundcube/installer
If you go to that URL, you will see the message:
"The installer is disabled! To enable it again, set $rcmail_config['enable_installer'] = true; in RCMAIL_CONFIG_DIR/main.inc.php"
So login into your server as root, open the file main.inc.php and change the line $rcmail_config['enable_installer'] = true; into $rcmail_config['enable_installer'] = false;
Now point to the installer URL again at http://www.example.tld:81/roundcube/installer
The RoundCube installer tells you anything wich is wrong and why RoundCube might not work.
Important: After changing your server configuration, lock the installer tool again!