Did you'll figure out the root cause for this problem? I have the same issue on my VPS (Ubuntu 8.04 & ISPConfig 18.104.22.168). After an indefinite period all outgoing traffic is blocked. I end up with the pretty much the same iptables rules posted in the first post in this thread. I dont remember setting up the iptables with these rules. Does ISPConfig write anything to the iptables? I tried flushing the iptables, but that crashed my VPS.
So any help/input is appreciated.