(I wonder, I have AirPort Extreme AC, too.)
well here are the steps I took. you may laugh at this as this sounds like the good old days when you had to hit your TV several times for it to start working.
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1. Did a factory Restore, just to make sure...
2. connected the Apple TV via WiFi, (the airplay still misbehaved over Ethernet at this time so i left it on WiFi sleep & airplay worked great while on WiFi, also i did log into all the services like store, home share etc.)
3. leave the apple TV on WiFi and let it go to sleep (I left mine over night)
4. without waking the apple TV plug in your Ethernet cable. I left this one in and constantly checked if the airplay service came up while the tv slept... so i left it sleeping over night once again.
5. Now I have a fully functional Apple TV
6. did a restart, did a unplug and everything works so far...
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This does not make any sense, all I can say is that the update was not smooth for me and this is what fixed it for me, let me know if this works for any of you guys.