To begin resolving Cloudflare error 521, ensure that your source web server can reply. Inspect the source web server log file for any web server program malfunctions or disruptions. Check to see if any Cloudflare IP addresses have been blocked or throttled. Activate all Cloudflare IP ranges in your originating web server's firewall or other security software. Check that you have a Cloudflare Origin Certificate with SSL/TLS mode set to Full or Full (Strict).