I also ran into the "Unable to connect to the MKS: Connection terminated by server" trying to open the console via vCenter.  I'm building a new 5.5 test environment and everything has been running mostly idle for ~2 weeks and when I went to access the consoles of the 4 VMs I have, none worked.

As a workaround, I did notice that I could use the vSphere client, connect to the host directly and open the console of the VM.