5xx Server errors · Cloudflare (not part of the HTTP standard)
520 Web Server Returned an Unknown Error
The origin server returned something empty, unknown or unexpected to Cloudflare.
Common causes
- The origin crashed or closed the connection.
- Response headers too large.
- Firewall on the origin blocking Cloudflare.
How to fix it
- Check the origin server's error logs.
- Make sure the origin's firewall allows Cloudflare's IP ranges.
Find out the moment your site returns 520
Spot Downtime monitors count this as down and alert you, since visitors see an error. (The public “Is it down?” checker treats 4xx answers from big sites as online, because many refuse automated requests.)