In my case the old login didn't work either.
When i chose to "sign up" again, the username was automatically inserted to email slot, and i just thought the form wanted me to double enter user name because it asks user name on the first slot aswell.
Needless to say this didn't work and i thought the forums are completely broken.
I used latest firefox as browser.
Then i switched to chrome, and in the sign up page i noticed i was supposed to enter email to the space that previously had user name automatically filled in, when using firefox.
I entered the information properly and all is working good.
So if your browser is playing tricks on you, Sign up page's correct form is: