It is my understanding that Xbox port forwarding is for routers that are not "compatible", don't have UPnP and require port forward. Modern routers with UPnP, eliminates the need for forwarding. I have 4 Xbox’s going at most times. How could I forward the same port to 4 different static IPs? Can't. Thus, you don't need port forwarding.
If you insist on port forwarding, you have to assign, whatever the device is, a static IP. Otherwise, with a dynamic IP, the forwarding would go somewhere else on the LAN. That sounds like why the dreambox acts like it does - not assigned a static IP. IMO, when you get rid of forwarding, it works like it should due to UPnP.