I have made researches and found out, that Twonkymedia is at fault. This is what they have reported in the XBMC forum also. (Citation: '...the xbox can't send multicast, so we send unicast (or actually broadcast)...'). It is obviously not working because of the missing broadcast support.
Twonkymedia Support gave me the following answer in case of a linux server is not detected:
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Use the -ip commandline switch to specify the IP address of your NIC. The ifconfig command prints it (usually for device eth0).
Check if you have the multicast route added:
route | grep 240.0.0.0
If the output is empty, add the multicast route:
route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0
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If I try this via Telnet on my WL500gP I get as message: NO SUCH DEVICE.
So what am I doing wrong? Please help!!