Things to check in general when this happens (I don’t know if any of them are relevant in this particular case):
Is Javascript enabled? Including third-party sites?
Are cookies enabled? Including third-party cookies?
Are you blocking or spoofing the referer?
After turning on scripting, cookies, and referer, open the registration page again, by clicking a link or copypasting the URL rather than by refreshing the page.
Things to check in general when this happens (I don’t know if any of them are relevant in this particular case):
Is Javascript enabled? Including third-party sites?
Are cookies enabled? Including third-party cookies?
Are you blocking or spoofing the referer?
After turning on scripting, cookies, and referer, open the registration page again, by clicking a link or copypasting the URL rather than by refreshing the page.