502: Bad Gateway
Likely cause
The origin returned an invalid response to whatever is in front of it — Cloudflare, or the origin's own reverse proxy/load balancer.
Check
Check the origin's own reverse proxy/load balancer logs (e.g. nginx, HAProxy) for the upstream connection that failed, and check whether the actual application server process is running.
Fix
Restart or fix the failing upstream application server/process, or correct the proxy's upstream configuration if it's pointing at the wrong address.
Verify
Retry the request and confirm it now reaches the application successfully.