It's just happened to me for the first time too, I've been in chat for the past couple weeks without any problems but I was in chat then bang it booted me for some reason, and now it just keeps asking me for username/pass etc ... which I enter, but then it just re-asks me for it.
I tried with an alternate browser but that behaved exactly the same.
Bignugget said rebooting his computer fixed the problem for him, but I can't reboot at the moment
Seems there's a bug in the chat software
I've been in discussion with the software authors for the past couple of weeks regarding a vulnerability so I'll see what they've got to say about this problem too, as I'd assume they would've heard of it before