I'm familiar with that error. I've seen it before (on Tiger), and it was my router bricking up. (firmware issue) For some reason the router would stop responding on certain ports - and cycling power to the router fixed it. It only happened every other month, if that.
I know 2 other people using D-Link routers (given to them for "free" with FIOS installs) and neither of them could get ichat to work for file transfers, video or audio. After unsuccessful attempts setting up one of the D-Links to work with Ichat, I copped out and replaced both with Linksys. (what can I say, it's the devil I know) FWIW, they all work now.
Forgot one more thing! I have had intermittent issues with Ichat when stealth browsing was on - turning it off on one of the macs fixed it. I can't find a rhyme or reason for this - but toggling it did help.