Error 520 through a proxy: the site’s server, not your proxy

Proxy error · Last revised Oct 19, 2026

Check that it is Cloudflare’s 520

Print the response headers through your proxy:

curl -s -D - -o /dev/null -x "http://USERNAME:[email protected]:8000" https://the-site-you-need.com/page

A Cloudflare error carries server: cloudflare and a cf-ray header. If the 520 comes without them, it did not come from Cloudflare: read your proxy provider’s own list of error codes instead.

What causes it

Cloudflare lists, among others: “Origin server crashes or misconfigurations”, “Firewalls or security plugins blocking Cloudflare IPs” at the origin, “Headers exceeding 128 KB (often due to excessive cookies)”, “Empty or malformed responses” and “Incorrect HTTP/2 configuration at the origin server”. All of them are on the site’s side. One of them can involve your request: headers over 128 KB. If your scraper carries every cookie the site ever set, the request can grow until the site’s server refuses it.

What to do

1. Load the same page without the proxy. If it fails there too, the site is broken for everyone; wait and try later.
2. Clear the session’s cookies and retry once. This rules out the one cause your request controls.
3. Retry with a wait. Crashes are often brief. Wait a minute, then a few, rather than retrying at once.
4. Do not rotate or switch plans to fix it. The address you send from does not reach the part that failed. Rotating spends traffic and gets the same 520.

If the site is yours, Cloudflare’s page gives the owner’s steps: “Contact your hosting provider or site administrator”, review the origin’s error logs, and check its HTTP/2 setup. Cloudflare also says: “If you are a site visitor, report the problem to the site owner” (Cloudflare 5xx errors).

Related

521 means the site’s server refused Cloudflare outright; 522 and 524 are timeouts behind Cloudflare. All the codes, sorted by who sends them: proxy errors.