Error 520 occurs when the origin server returns an empty, unknown, or unexpected response to Cloudflare.
Contact your hosting provider or site administrator and request a review of your origin web server error logs for crashes and to check for these common causes:
- Origin web server application crashes
- Cloudflare IPs not whitelisted at your origin.
- Origin web server TCP idle timeouts shorter than 300 seconds
- Headers exceeding 8 KB (typically due to too many cookies)
- An empty response from the origin web server that lacks an HTTP status code or response body
- Missing response headers or origin web server not returning proper HTTP error responses
If 520 errors continue after contacting your hosting provider or site administrator, a web developer should provide the following information to Cloudflare Support:
- Full URL(s) of the resource requested when the error occurred
- Cloudflare Ray ID from the 520 error message
- Output from http://www.example.com/cdn-cgi/trace (replace www.example.com with your hostname and domain where the 520 error occurred)
- HAR file with Cloudflare enabled on your website
- HAR file with Cloudflare temporarily disabled
Source: https://support.cloudflare.com/hc/en-us/articles/115003011431#520error