Firewall/NAT

Hi,

I've been using monowall for a couple of years now without issues.
Recently, my xbox and pc can't find eachother anymore.
Here's what happened

Before, i didn't change any default settings, apart from a dhcp recervation for my xbox. everything worked ok.
I bought a hdd for the xbox, installed it, and the xbox automatically downloaded some updates

Since then, the pc can't connect to the xbox anymore. I tried factory settings for the xbox, adding nat rules and firewallrules for the xbox, no use.
The pc can see the xbox in the network, but wmc won't install an extender, since it can't find it.

The xbox doesn't get an ip anymore (except when i "test" the connection, then it gets it without problems)

Xbox support can't tell me anything, they point me to windows support (and we all know how fast and good they are)

When i test the connection to xbox live (which i don't really use) it gives me that my nat is set to medium, and that i should enable upnp (which i don't want for security reasons, apart from that monowall doesn't support upnp)

My network atm is the following

1/ modem-router provided by the internetprovider, monowall is in dmz, all ports are forwarded to dmz
2/ monowall 1.33
3/ switch for all of the pc's, including the xbox

No av or fw on the pc active or installed

Anyone who can point me in the correct direction here?
If more info is needed, just ask :-)

It appears to me that it's an xbox related problem, because it occurred right after the automatic xbox upgrade.