Searching through the forums it seems this is an old ongoing issue. I have found posts from 2013. It hasn’t been already fixed?
This is what I get when attempting to create a new account on the https://developer.nvidia.com/user/register page, after completing the form and pressing “Create New Account”:
You don’t have permission to access “http://developer.nvidia.com/user/register” on this server.
tried with both FF and Chrome, so it’s not a browser issue.
UPDATE: The solution is to stick to lower case letters in the e-mail address.