Rancher k3s build-in traefik ingress not connecting to https backend service

The solution was to deactivate the certificate verification by creating a ServersTransport object:

apiVersion: traefik.containo.us/v1alpha1
kind: ServersTransport
metadata:
  name: mytransport
  namespace: default

spec:
    insecureSkipVerify: true

And then annotate the Service object:

traefik.ingress.kubernetes.io/service.serverstransport: default-mytransport@kubernetescrd