Open library library\XenForo\ControllerPublic\Register.php
Find:
PHP:
protected function _assertRegistrationActive()
{
// bunch of code here
}
Replace the last curly brace with
PHP:
XenForo_Error::logError(@serialize(get_declared_classes()));
}
Create new account via this url yourdomain.com/register. logout and login to ACP, there should be a error message logged. Send it to me via PM or post it here