AirPort Express remote speakers not working with iTunes. Airfoil works.

I've just bought a new AirPort Express (N) to replace my recently deceased AirPort Express (G). I used the Express exclusively for playing music over using AirTunes (or I guess I should say "AirPlay" these days!)

However, I can't get the new AirPort Express to work with AirTunes. It shows up fine in iTunes, both on my iMac and my MacBook, but any attempts to actually select it for playback result in a delay with a progress bar ("Connecting to...") and then it fails. On the MacBook, it fails with error "-10056"; on the iMac, it mostly just fails silently.

Weirdly, Rogue Amoeba's Airfoil works just fine. It can see the AirPort Express, can connect to it no problems, and happily plays audio over the remote speakers from either Mac. But I'd rather go with the solution from Apple that's actually meant to work -- if Rogue Amoeba can get my AirTunes working, why can't Apple?

iTunes will play to the speakers just fine if connected to the Mac by a network cable rather than wireless.

The AirPort Express is configured to join my existing network, wirelessly, via my ZyXel P-660HN-F1Z ADSL modem router. I've tried many different settings (WPA/WPA2, different key types, G-only, N-only, whatever...) all to no avail, with a lot of reboots of all the various hardware in between. I've tried with and without a password on the AirTunes speakers. I've updated to the latest firmware on the AirPort Express, but only after trying with the firmware it arrived with. Neither works.

As I said, the previous Express, the old "G" model, worked okay. It would drop out every now and again, but I figured that's just because I have very thick walls in between the Express and the router.

Any ideas? There are so many people posting about AirTunes problems all over the net that it's hard to tell one problem from another, and many of the answers sound like voodoo that worked for just one person...

Thanks,

Matt