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

5xx · Server errorNot part of the HTTP standard

HTTP 520 Web Server Returned an Unknown Error

Cloudflare: the origin sent an empty, unknown or unexpected response.

Example response

HTTP

HTTP/1.1 520 Web Server Returned an Unknown Error
Content-Type: application/json; charset=utf-8

{ "error": "Web Server Returned an Unknown Error" }

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

What does HTTP 520 mean?: Web Server Returned an Unknown Error. Cloudflare: the origin sent an empty, unknown or unexpected response.
Is 520 an error?: Yes. 5xx codes are server errors: The server failed to fulfil a valid-looking request.