Metrics collection from Kubelet API (stats/metrics) failed

We are running Rancher v1.6.12 with a Kubernetes cluster (v1.8.x). I deployed Telegraf as a DaemonSet to collect metrics from endpoint on each node but it does not work.
I tested directly on the nodes with wget -q -O -, got back:
wget: server returned error: HTTP/1.1 500 Internal Server Error

Although /stats/ gave back metrics about the host.

I have checked the summary API on Minikube and on GKE and those were working fine. I thought that Kubelet API is the standard to collect these kind of data, I was googling for a hint but found nothing related, did I miss something?