[Problem]
In our setup i have a ASUS uPNP media player that is attached to a sub router on the network, which happens to also be the WLAN router. Every time i closed my notebook running Win7, the Asus box stopped playback and threw me into main menu, even tho the two devices do not have anything in common for playback (playback server is a totally different machine).
So... why the hell is putting a notebook in stand-by interrupting the movie playback on a stand-alone uPNP player?
[Solution]
The notebook ran Win7 which by default shares media to the network using uPNP. Every time the notebook was shut down or going into stand-by, this service would broadcast something that fucks with the uPNP media devices on the same subnet. Sooo what to do? As the notebook never was used as mediaserver anyway, there is no need to have the service for that running - so disable the service "Windows Media Player Network Sharing Service"; put it to manual start-up and then stop the service so it won't come up on next reboot again. Only start it if you really need it. Problem solved.