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