What the error actually means, and whose problem it is.
Each of these is a mechanically different failure, even when the symptom looks similar. Find yours below for a plain-language answer, plus a real diagnostic checklist if you're the one who runs the service.
504 Gateway Timeout
The proxy in front of the backend gave up waiting for a response — a timeout, not a crash.
503 Service Unavailable
The server is deliberately declining requests right now — maintenance, overload shedding, or a not-ready health check.
502 Bad Gateway
The proxy couldn't get a usable response from the backend — usually a crashed process or a bad upstream address.
ERR_CONNECTION_REFUSED
Nothing is listening on that port at all — a TCP-level failure, before HTTP even enters the picture.
DNS_PROBE_FINISHED_NXDOMAIN
DNS resolution itself failed — the domain doesn't resolve to anything, a layer below HTTP.
SSL/TLS Connection Error
The certificate or the handshake itself failed — the browser refuses the connection before HTTP even starts.
More coming as the reference grows. · Looking for a solution instead of an error? →
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