502 Bad Gateway when backend unreachable despite router match

Router matches and rule is correct, but Traefik returns 502 Bad Gateway. Backend container is unreachable due to: wrong Docker network, incorrect service port, application listening on 127.0.0.1 instead of 0.0.0.0, or container in restart loop. Service may show no available backends in Traefik API.

Test connectivity from Traefik container to backend with 'docker exec -it traefik sh -lc "wget -qSO- http://<backend-ip>:<port>/health"'. Verify service port in Traefik API matches application listening port. Check application binds to 0.0.0.0, not localhost. Verify container is running and healthy with 'docker ps'. Ensure both containers share a network. Set explicit loadbalancer.server.port label. Check application logs for startup errors.