Temporarily deactivate all plugins except Restrict Content Pro. Now try the registration form. If it works, that means there's a plugin conflict. To identify the problematic plugin, start activating the plugins one by one until the registration form breaks.
If the registration form does not work with all the plugins deactivated, leave them deactivated and proceed with the theme section below.
First, if you've customized the registration form in any way, as outlined in our instructions here, remove those customizations. If the form works, it's a problem with your customization. We do not provide custom development support.
If you have not customized the registration template, temporarily switch to the default theme that comes with WordPress. At the time of this writing, the default theme is the Twenty Seventeen theme.
Still need help?
Open a support ticket here.