sorry for the delay. It took me a while to find out that what this panel is operating is no more than a simple Iptables ruleset.
I stopped Iptables, and ISPConfig installed without a problem.
But I lost my original customized ruleset in the process of experimenting on panel level, and I now have 20 rules less than before which I will hardly be able to reconstruct. On top of that, I still don't know exactly what went wrong with the MySQL rule before.
So my question: Is there an IPtables ruleset known to work well with all aspects of ISPConfig that I could download/copy from somewhere?