AirPlay error (-15000)

I have an AirPort Extreme that is connected via Ethernet and Wi-Fi to my iMac. Am am trying to AirPlay my stereo connected to the headphone jack on my AirPort Express that is extending the 5GHz network on the Extreme. I have been doing this without any major problems for the past few weeks (apart from hiccups and pauses), up until today. Everytime I try to AirPlay the AirPort Express, iTunes returns the following message while attempting to connect. "An error occurred while connecting to the AirPlay device “AirPort Express”. An unknown error occurred (-15000)."
I can confirm that the AirPort Express is well within range and is functioning correctly extending the 5GHz network. I have checked to see if my radio channels are conflicting with any nearby networks, and it is not. Any help?

Thanks,

Drew

Running:
AirPort Extreme Version: 7.6.3 (802.11n 5th Generation) - 5GHz Channel on 36, 2.4 GHz Channel on 10
AirPort Express Version: 7.6.2 (802.11n 2nd Generation) - 5GHz Channel on 36, 2.4 GHz Channel on 11
iMac running Mountain Lion 10.8.4
iTunes Version: 11.0.4
Motorola Surboard SB6121, DOCSIS 3.0 Modem with Comcast Blast! Internet Service (roughly 66mbps download, 11mbps upload)