I wondered if you might possibly be typing your username with the wrong combination of uppercase and lowercase, but no. I just tried logging in as "fooze" and the system welcomed me as "FooZe".

Any chance you might have a stray cookie on your computer with a variant spelling or something? Next time you're logged in, try logging out (which is supposed to clear your cookies), then see if the problem comes up again when you log back in.

I suppose you could even try clearing all your browser cookies just to make sure that wasn't it.