I am on Denon's latest Firmware for my 4311.
I am on OSX 10.9.5 on my MBP Retina (iTunes 12), via WLAN, 10.9.5 on my iMac (iTunes 11), via Ethernet.
I am on IOS 8.1 on my iPad Air and Mini Retina.
I am on IOS 7.x on my iPad 2.
None of the devices could connect any more for more than fractions of a second to the Denon before I rebooted the Time Capsule.
I rebooted my Time Capsule (only used as WLAN bridge using IP4), latest firmware.
I rebooted my main LAN Switch where the Time Capsule and the Denon are connected to (Denon through an additional switch).
I powered off (unplugged mains cable) my Denon.
Switched everything back on (tried any order, and more than once).
After I rebooted the Time Capsule, the MBP Retina (running iTunes 12) was able to connect (exactly once), but after pausing the music I was listening to, everything locked up again for any device - even rebooting again everything didn't help any more this time. Now when carefully watching what's happening, I can see my Denon changing the volume sometimes from -60dB to -6dB or -infinity (unpredictable), sometimes it doesn't change the volume when I switch any of the Devices to Airplay on the Denon. The Denon always tries to switch to AirPlay, but falls back to the NET source menu immediately: not a fraction of sound audible before falling back.
AirPlay works just fine against Apple TV 3 (?, the 1080p box) with its latest firmware from any device.
I am a bit cueless. Sometimes I suspect the latest Denon firmware update (a few months? weeks? ago), sometimes I suspect the apple stuff. So much for "originally licensed by" and believing that living in the apple universe would make things a little bit less troublesome as they used to be in other universes, like DLNA for example...