Hm, that's odd. Sorry for that.
Please try to reset your old password. To do so, please go to settings and open your user profile. Enter your email address and click "Forgot your password" and we will send you a new password to this address.
If this does not work, please click "Report a new problem" and tell us, how you try to register and which email address you are using for that. We try to solve your problem as quickly as possible.