To begin fixing Cloudflare error 521, make sure your source web server is able to respond. Examine the source web server log file for any breakdowns or interruptions in the web server application. Verify that Cloudflare IP addresses aren't banned or throttled. In your originating web server's firewall or even other security software, enable all Cloudflare IP ranges. Verify that you have acquired a Cloudflare Origin Certificate with SSL/TLS mode at Full or Full (Strict).