THIS ANSWER IS WRONG. The correct version is that Foldit automatically adjusts your chosen user name to make a valid IRC nickname. This happens only for your first user name. If you change your Foldit user name, your IRC nickname will remain the same. If you want to change your IRC nickname, you must ask the Foldit team to do it. It's a manual process.
I've crossed out the wrong parts of the answer. The other information is true, but probably not relevant to any chat problems we're seeing at the moment. Sorry for any confusion I caused with the wrong information.
The problem may be your Foldit user name.
According to chat specs, IRC nicknames can be only nine characters long, and cannot contain a period or full stop (".").
The Haykapnayan players I see posting in global have names longer than 9 characters, but the names don't have any dots.
If your still having problems getting into chat, I'd suggest changing your Foldit user name. Log in to fold.it, go to "My Page", then click on "Edit". Then, you can remove the dots, and click "Save" at the bottom of the page.
For reference, here are the rules from IETF RFC 2812:
nickname = ( letter / special ) *8( letter / digit / special / "-" )
letter = %x41-5A / %x61-7A ; A-Z / a-z
digit = %x30-39 ; 0-9
special = %x5B-60 / %x7B-7D ; "[", "]", "\", "`", "_", "^", "{", "|", "}"
That says the first character of your IRC nickname must be a letter, A-Z or a-z, then up to 8 more letters, digits, or certain special characters, including a hyphen or dash ("-").
The spec also doesn't allow spaces in an IRC nickname, and doesn't even begin to allow Unicode. It's very 7-bit ASCII.
Clearly, the length limit is not enforced, but I don't see any current IRC users with "." or even one of the other "special" characters in their names. (This is because Foldit automatically removes "." and other invalid characters from the user name to create an IRC nickname. Some users do have characters like underscore ("_") in their names.)
See also Password incorrect bug from a couple weeks ago here in the forum. In that case, the player had special characters in their password. The website accepted the special characters, but the Foldit client didn't. Not exactly the same issue you're seeing, but kind of similar. The Foldit website may allow characters that aren't available in the actual game. Thanks to HuubR for correctly diagnosing the issue on that previous problem.