I've tried the Solspace User module registration form and the standard EE registration form and I'm get the same error on both submissions:
The form you submitted contained the following errors: You must provide a valid registration code to complete your registration. (note: Registration codes, like passwords, are case-sensitive.)
I have no reference to registration codes or even a captcha in my registration forms. In my member preferences I have allow member registrations set to yes, require terms of service is no, enable membership captcha is no.
I'm not sure what is wrong and what registration code EE is looking for?
Any help would be very much appreciated.