This is an application error, not from Traefik.
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Enable Traefik access log in JSON format (doc) to get more details about the request, for example which target service instance is used.
Alternatively you can use docker compose logs -f to see all your container logs at the same time.
Furthermore share your full Traefik static and dynamic config.
My personal recommendation is to use explicit Docker networks, especially if you scale in the future.