521 Web server is down
HTTP response status code 521 Web server is down is an unofficial server error that is specific to Cloudflare. This HTTP status code occurs when the HTTP client was able to successfully connect to Cloudflare but it was unable to connect to the origin server.
Cloudflare is a content delivery network that acts as a gateway between a user and a website server. When the 521 Web server is down status code is received, Cloudflare attempts to connect to the origin server on port 80 or port 443 but the HTTP Connection is refused by the origin server.