This looks like the access to admin-ajax.php is forbidden (error 403). For some reason, many security plugins are preventing access to admin-ajax.php, even with this file being essential for WordPress to work correctly.
- If you use some security plugin, check out if it is maybe blocking admin-ajax.php.
- Check if admin-ajax.php is blocked in .htaccess file (if you use Apache server).
- Check with your hosting company to examine logs and find out what is blocking admin-ajax.php access.