Too much work at work to do have any Mulle progress

At work I am trying to set up a two-ISP failover firewall/bridge combination using iptools2 and netfilter on a GNU/Linux box (using Debian now :) ). Works ALMOST with the exception that for whatever reason UDP packets from one(!) server of our local Class-C net are dropped by the ISPs router, but only if the firewalling bridge is in the middle. TCP traffic passes without problem.

Looks like a Linux problem so far ARGH! (Can't be my mistake now, can it :))