Please have a look here:
http://www.nabble.com/help,-can't-compile-clamav-0.92-(%22gcc-bug%22)-td14380236.html
Try to downgrade your gcc version to a version that dont has this bug by running:
apt-get install gcc-3.4
Before you try to update ispconfig again, run this command:
mkdir /root/ispconfig/
Update:
I added a post about a problem with possible solutions:
http://www.howtoforge.com/forums/showthread.php?t=19222