This is an error I often see at customers when trying to open a virtual machine console: “unable to connect to the mks”
The error is not so clear, but at 99,9% is due to incorrect DNS resolution, in other words….. if you have connected to the vcenter from a client, the client from which you have connected should properly resolve esxi hostnames as you see them in the vcenter. The correct thing would be resolve them by DNS, but if you can not edit them for any reason you can also work around the problem by inserting the resolution in the hosts file of the machine from which you got connected.
P.S.
The problem does not occur if you connect directly to a single esxi node so if you have urgency to open a vm console you can avoid the problem in this way.