5xx Server error · Non-standard, Cloudflare
520 Web Server Returned an Unknown Error
Cloudflare-specific: the origin server returned an empty, unknown or unexpected response.
Cloudflare reached your server but the response was invalid — for example the connection was reset or the headers were too large.
Common causes and fixes
- Check origin logs for crashes or resets at that time.
- Make sure response headers (including cookies) aren’t excessively large.
- Confirm the origin isn’t blocking Cloudflare’s IP ranges.
Example response
HTTP/1.1 520 Web Server Returned an Unknown Error
Date: Thu, 22 Oct 2026 08:00:00 GMT Check a URL’s status with curl -I https://example.com, which prints the status line and headers.