I faced a issue with one of my clients sites after migration. The facebook debugger was returning http response code as 404 , even though the webpage was loading fine. No issues with meta tags. After hours of research its the htaccess file which was creating the issue. The site had bullet proof plugin installed and we deactivated and uninstalled it but the htaccess had traces of BPS.

So cleared htaccess and voila every started to work fine.