Can't connect to AirPlay

I just added a second AirPort Express (latest, second generation) to our network and am trying to get our Mac Mini to play through the speakers attached to the AirPort. No joy. I'm able to connect from a MacBook Air (via wireless connection) and from a MacBook Pro (connected via Ethernet). The Mac Mini is using a wired connection. Both of the MacBooks are running 10.7.3, the Mac Mini is running 10.6.8. All are using iTunes 10.3.

The symptom that I'm seeing is that I can see the AirPort Express speakers but can't select them from the Mini. I can select them and deselect them from the MacBooks (and I've taken care to make sure that none of the other machines are using the speakers when I try and connect with the Mini. On the Mini I see the following message in Console: AppleGVA:: Error creating the accelerator 1. Though not enough instances to account for all of my attempts to connect to the remote speakers.

For whatever it is worth, this used to work…

I've removed and reinstalled iTunes on the Mini, blown away the preferences, and am considering making ritual sacrifies. Any suggestions would be most appreciated.