We're performing an account migration on mangband.org, and you're probably going to experience problems with forum login.
You'll have to sign up for mangband account. Make sure you use your forum username. After you're done you should be able to login here using your newly accuired "mangband account".
I left other hints along the process too. If you have any questions feel free to ask here, or on IRC (incase you can't login here).