Another short one.
This happened to me in version 5.5. I set a host in maintenance mode in a cluster, and all VMs moved, but when entering maintenance mode the host stalled at 68%.
KB 1002412 states that this is either a vcenter problem or a time problem. I offer a 3rd solution.
Connect to the host through drac and restart the management agents. The host will disconnect and reconnect, and now entering maintenance mode will work ok.