Update: It seems after a reboot, I have to wait for some (3-5) minutes and then suddenly my firewall rules are obeyed. Prior to that, nothing works (except ping, which I have no rule for). This delay of course confuses my testing as I expected immediate results from firewall changes.
I did install the "dbus" package and all my dbus errors have vanished. I suggest to add dbus to your list of dependencies. Installing dbus however didn't solve anything except for the displayed error.
The last remaining error is that I have to run "sudo dhclient br1" after each boot if I want any nameserver usage with ubuntu/zentyal. I don't know why ubuntu/zentyal fails to setup nameservers (/etc/resolv.conf is always empty upon boot but is filled (correctly) with 'sudo dhclient br1').