Errors trying to connect to Kubernetes web UI
You could check the similar issue on GitHub.
Do you execute az acs kubernetes get-credentials --resource-group=myResourcrGroup --name=myK8sCluster on you client PC? I suggest you could check your private key and execute the command again.
You could use azure -v to check your Azure CLI version. My version is 0.10.9. If it does not work again, I suggest you could re-install the latest version Azure CLI.