Unverified50% confidenceFactExact time
HTTP 521 is a Cloudflare-specific error code indicating that the origin server refused the connection, typically meaning the origin is down or refusing to respond.
1
Sources
50%
Confidence
Long-term
Relevance
10/12/2026
First Seen
Sources
Related Entities
Related Claims
UnverifiedHTTP 503 indicates the server is temporarily unable to handle the request, 500 indicates a backend logic anomaly, and 404 indicates that the requested resource does not exist.72% similarUnverifiedHTTP status code 500 (Internal Server Error) indicates that an unexpected error occurred on the server side; the specific cause needs to be identified by combining server logs and code logic.71% similarUnverifiedHTTP status code 401 indicates authentication failure, 429 indicates rate limiting has been triggered, and 500 indicates a server-side error.68% similarUnverifiedThe distribution of failure types was 344 HTTP 503s, 103 HTTP 504s, 80 HTTP 521s, 52 timeout interruptions, 18 502s, 7 500s, and 1 520.65% similarUnverifiedWhen a rate limit dimension of an API provider is hit, the provider returns an HTTP 429 error.65% similar
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