12.04 Hyper-V client is not routing packets

I have got one Hyper-V Server with several guests running.

One of them is a Ubuntu 11.10 which tasks are routing and openVPN. The virtual router have got 5 NICs.
  eth0: LAN (standard external NIC from Hyper-V)
  eth1: internal network card (so not "mounted" to a physical card)
  eth2: another internal network (currently not used)
  eth3: IPTV network (mounted to a physical LAN)
  eth4: connection to the VDSL modem (so it is a physicaly mounted NIC)
  ppp0: internet connection
  tap0: openVPN

So as you can see it is just a router. Everything is working as expected with the 11.10 machine.

Before I have ported (fresh reinstall with manual configuring based on the old machine) this machine some times - so I am farmilar to the procedure.
I have noticed that the Hyper-V drivers are build in and are not needed to be activated through the initramfs-tools/modules as before needed.

The new machine was working so far. After I was finished installing the basics I copied the interfaces configuration to copy the ifconfig of the old host. I shut down the old machine and rebooted the new server. Everything looked fine, I could connect through ssh and ping the server. The server was connected itself to the internet. (ping to 8.8.8.8 was working).

Now the problem.
The router is not routing. This is the best discription.
I am sure that this is not an iptables problem (I have copied the script); afaik NAT problem because internal routing is not working, too. My gaming PC can ping the router but it was not able to ping to 8.8.8.8 neither to one of the other internal LANs. But the machines which are connected to the internal LANs can ping the router but not other components which are connected to other systems.

Example:
Router: 172.16.1.17/28 (eth1); 172.16.0.1/16 (eth0)
Server1: 172.16.1.20 /28 <-- in eth1
Client1: 172.16.0.10 /16 <-- in eth0

Server1 can ping the router (ALL addresses). So it even can ping the 172.16.0.1. But it cannot ping the Client1.
Taking a look to tcpdump only shows an incoming echo request from the eth1 but no outgoing at the other interface.

To summerize: Only one network can be reached - no routing through the different networks.