Hi to myself
I tried to get a little deaper. I started TCPDUMP on the server PC and there where multicast packets send by the server and also send by Noxon. That's the reason why I can see the Noxon in the status site of the TwonkyServer.
I also started Whireshark on my notebook. It's like the Noxon connected via WLAN, so I expected to see the same multicast packets (shared medium). But there where only multicasts from the Noxon and no communication from the server.
Is there an error in reasoning? Where are the multicasts from the server?