Ah, I had the same problem some weeks ago!
It seems that, old accounts in the bug tracker were username/pass defined whereas it was changed to email/pass instead some time ago.
I remember me typing my email/pass trying to login (or to use the email password feature) a lot of times until I discovered it.
So, just now, some registered users (older) continue using the username and other (newer) have to use the email.
Anyway, it's mandatory for new accounts being created to use the email (and it is working just now).
Ciao