I'll try to check this out - I'm wondering if it could be to do with the log-in page(s)?
Here's a theory which could be totally wrong but.........I gave a link to the log-in page for first-time users to register - I usually give another link. Both links work fine for me ( I use i-chat), but please would you check both links and see if one of them works for you?
I had the same problem but next to the login button there is a box that says default - choose another option, I think I choose i-chat and it should let you in.