502 5xx RFC 9110 §15.6.3

Bad Gateway

Server (acting as proxy/gateway) received an invalid response from upstream.

What HTTP 502 means

RFC 9110 §15.6.3. The reverse proxy (nginx, Cloudflare, ALB) couldn't get a valid response from your application. Causes: app crashed, app not listening on expected port, malformed response from upstream, upstream connection reset.

Typical causes

Upstream app crashed
App not listening
Connection reset by upstream
Malformed upstream response
Wrong upstream port