Have a look at this discussion. It will explain everything.
thanks a lot thunder. this helped me for now, but it's just a work around as still the registration doesn't work. i don't understand why it worked and now suddenly it stopped????
You're quite welcome.
Removal of Ning ID has been a massive undertaking for Ning. The fact that you have not had this problem previously just means it's your turn. Until they completely remove it, these sorts of things will continue to occur.