https://github.com/kubernetes/kops/issues/964

Did some digging around and found that it is because of self signed certificates. This can be solved by adding

--insecure-skip-tls-verify=true 

to every kubectl command or (the preferred way) adding:

--kubelet-certificate-authority=/srv/kubernetes/ca.crt \
--kubelet-client-certificate=/var/run/kubernetes/kubelet.crt \
--kubelet-client-key=/var/run/kubernetes/kubelet.key 

to the kube-apiserver startup shell script.