Why is the K8s UI not available when rancher is used?
I've seen this happen, especially on the newest (as of now) version. Try going into the "Infrastructure stack" and look for stopped containers. Often it helps to just refresh the entire deployment (hit the button "up to date" next to the "kubernetes" stack and hit save (If I remember correctly) in the bottom to force the "refresh").

You could also try manually just restarting the kubernetes-dashboard containers.

Lastly you could use the kubectl cli to delete the dashboard pods and force them to get recreated.

Let me know how it goes! I know I had to do this fix a few times.