520: Web server returns an unknown error
Did this happen on one specific page or endpoint, or across your whole site?
One specific page or endpoint
Cloudflare received a response from your origin that didn't look like a valid HTTP response for this particular request.
Check
Reproduce the failing request directly against your origin (bypassing Cloudflare) and inspect the raw response headers/body.
Fix
Fix the malformed response from your application code (e.g. missing/invalid headers, a crashed process mid-response, or a proxy misconfiguration on that route).
Verify
Retry the same request through Cloudflare and confirm it now returns a normal response.
Across the whole site
Something at the origin — the web server process, a reverse proxy in front of it, or the app server itself — is returning responses Cloudflare can't parse for most or all requests.
Check
Check your origin web server's error logs for crashes or malformed-response warnings around the time this started.
Fix
Restart the origin web server/app process and review its configuration (e.g. proxy_pass settings, response buffering) for anything recently changed.
Verify
Retry a few different pages through Cloudflare and confirm none return 520.
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