after the old domain name was switched over, the recaptcha key stopped working and new users could not register anymore. Recaptcha.net requires that one specifies all domains that a key is going to be used from, probably to prevent abuse. I have now added registry.gimp.org and my testing indicates it works fine again, so you should be able to create new accounts again!
Many thanks to the people who reported this in a very helpful manner (with screenshots, even!).