Question
Why do I see a 521 error when attempting to view my website?
Answer
This error can sometimes occur when CloudFlare attempts to reach your server on port 80 or 443.
To correct the issue, you can whitelist CloudFlare's IP's so they can establish a TCP connection to your server over ports 80 and 443.
For more information, please see the following:
https://community.cloudflare.com/t/community-tip-fixing-error-521-web-server-is-down/42461