If you try to connect to a VM’s console from the VI Client and receive something similar to:
Unable to connect to the MKS: Failed to connect to server esx4.mydomain.com:903
Almost every time you see this it’s DNS. So check that first. Refer to this KB article too.
For me, I knew it wasn’t DNS as I did the normal tests and the KB article didn’t help. The problem started when I moved a customer’s vSphere cluster to a new management network. We created a new Service Console on each vSphere host in a new vSwitch on a new VLAN. Then we changed DNS to point to the new IP, removed the host from vCenter, and then added it back. Everything was fine, except for VM console. The fix for this is either to reboot the ESX host (preferably in maintenance mode!) or for a non-disruptive fix SSH in to the host’s console and type:
This refreshes the network configuration and allows you to connect again. Easy!