ACP - You've Made Too Many Attempts To Register
You've made too many attempts to register. Please try again later.
Goto ACP > Configuration > Server > Allowed login attempts: Change the number if applicable and Submit.
Goto ACP > Configuration > Cracker Tracker > Registration Protection System > Block Time for Registration: Change the number if applicable and Submit.
Thanks to nvram, this could also be the problem.
Using VARNISH script on your server will always take the users address as localhost and block the registration.