EDoesn't look like a Traefik issue to me. The asset could not be served error is a known issue that has nothing to do with the user not found error https://github.com/zitadel/zitadel/issues/4297. Did you try the username zitadel-admin@zitadel.auth.XXXX.io?
GitHub
ZITADEL - The best of Auth0 and Keycloak combined. Built for the serverless era. - Issues · zitadel/zitadel