Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.

5xx · Server errorNot part of the HTTP standard

HTTP 521 Web Server Is Down

Cloudflare: the origin refused the connection. The server is down or blocking Cloudflare.

Example response

HTTP

HTTP/1.1 521 Web Server Is Down
Content-Type: application/json; charset=utf-8

{ "error": "Web Server Is Down" }

Look up the headers shown here in the HTTP headers reference.

What does HTTP 521 mean?: Web Server Is Down. Cloudflare: the origin refused the connection. The server is down or blocking Cloudflare.
Is 521 an error?: Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.