520 Web Server Returned an Unknown Error
Meaning
A Cloudflare-specific status indicating that the origin server returned an unexpected or unknown response. It’s essentially a catch-all when the origin’s response is something Cloudflare doesn't know how to handle (not a standard HTTP response).
When to Use
Used by Cloudflare's reverse proxy service when the origin server responds with something that is empty or not recognizable as a valid HTTP response, or closes the connection unexpectedly. It tells the client that something went wrong but the details are not clear.
Example
If Cloudflare requests content from an origin server and the server responds with gibberish or an empty response (not even headers), Cloudflare will return a 520 error to the client to indicate an unknown error at the origin.
When Not to Use
Do not use outside of Cloudflare or similar proxy contexts. As a client or origin developer, if you see a 520, it usually means the issue is with the origin server's response, not a code you'd intentionally send. For standard servers, prefer using explicit 5xx errors.
Source
Unofficial (Cloudflare)