Apparantly it has something to do with not being registered properly. Cyanara had that problem, but managed to fix it and tell everybody how to. I didn't think to save how she did it, otherwise I would have posted it here.
...Wait, did you just call me a "she"? Uh, never mind. Anywho, time for: Cyanara's Tech Support Hour!!!~
I realised that I had never actually received an activation email, so I sought out a resend form and found one. unfortunately, it asked me for my username and email address, while only providing a box for my username, and when I tried just that it told me that no such user existed. Of course not. I'd only posted several times...
Moving on, I tried the lost password form. Hooray! All the fields were there. Filled that out. Got an email almost instantly. Logged in with the new password. Both forum and Chatroom acknowledged me and I changed my password back to something slightly memorable.
Next time on Cyanara's Tech Support Hour!!!~
Q. Why the fsck did the chatroom drop of the last letter in my name, when other people had even longer names?
A. Don't know! Too many Tafe assignments to do...