You're trying to access the registration save endpoint directly. That just processes the POST data of the registration page and it's throwing that error because it's missing the spoof check. Your registration page can be accessed with the below URL (click Sign Up in your login module top right of the page).

www.ocra-dg.org/index.php/cb-registration

I've tested your registration as working using username of "testuser" (feel free to delete the user). If you're unable to register using the above URL you may have a browser configuration blocking cookies, which are necessary for Joomla (and CB) to function.