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HTTP_STATUS_UNKNOWN_ERROR
Cloudflare reports 520 when it cannot classify the origin failure as a more specific HTTP error. Documented causes include origin crashes, firewalls blocking Cloudflare, oversized headers, malformed or empty responses, and broken HTTP/2 behavior at the origin.
Capture the Cloudflare Ray ID and response headers, then compare them with origin logs. Check origin response size, HTTP protocol implementation, and firewall policy before changing the client.
Cloudflare uses 520 when the origin interaction fails in a way that does not map cleanly to a more specific gateway code, for example an empty, malformed, or otherwise unexpected origin response. Check origin logs at the matching request time and validate response framing, headers, and server stability.
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