The following could be the cause of this, as I did the same installation and have no problems installing ISPConfig on my Ubuntu 6.10.
Ubuntu developers have replaced Bash with Dash, so the regular /bin/sh that is used to be linked to /bin/bash is now pointing to /bin/dash.
You can remove the /bin/sh and create a new link using:
rm /bin/sh (or if you want to be safe: mv /bin/sh /bin/sh.dash)
ln -s /bin/bash /bin/sh