Recently I come across an issue with instant clone pool. It was showing an error that “Cloning of VM has failed: Fault type is UNKNOWN_FAULT_FATAL – Either the resource pool: xx xx xx is empty or all its hosts are marked for maintenance”
This is because the hosts are in or are marked for maintenance mode.
Bring the hosts out of maintenance. Make sure that InstantClone.Maintenance is not set to 1 or 2 for any of the hosts.
If it is, from vSphere Client, go to the host’s Summary tab,
Edit Annotations, and clear the value for InstantClone.Maintenance.
You are done !!
More info refer VMware KB2144942