HTTP 521 – Web Server Is Down
(Cloudflare) The origin server refused the connection from Cloudflare, usually because it is offline.
Common use case
Appears when the origin web server process itself is stopped or crashed, so the TCP connection is actively refused rather than timing out.
History
A Cloudflare-specific extension.
Did you know?
Because Cloudflare sits in front of the origin, visitors see this branded error page instead of a generic "connection refused" - making outages far more visible than they'd otherwise be.