I built a new system, and had the same issue. After some googling, I found a fix.
There is an issue with clamav on Debian with the stable version. You need to reinstall clamav from Debian volatile:
Add the following to /etc/apt/sources.list:
etch/volatile main contrib non-free
apt-get remove clamav
apt-get install clamav
Mail forwarding and clamav now work properly on both of my ISPConfig boxes. Thanks for your help Tim!
Perhaps my findings will help someone else who is using Debian.