Just had this issue myself now in ESX 6.5, with all updates available as of 24/07/2018 installed.
The issue for me turned out to be that the 2 VMs affected for me had Virtual Optical Drives added, rather than any snapshots being present. The drives didn't have any ISO's loaded, and they weren't 'connected' to the VMs, but them being added to the config was enough.
As soon as I edited the settings of these two VMs, and removed the Optical Drives, i was able to move them to any other server in the farm.