CSV event log:
Mount copy,13749d25-48f7-4cbc-ad7c-b9d89571xxxx,VM_000086,XXXXX,Mount of Datastore failed with error com.vmware.vim25.PlatformConfigFault . Check the server.log for details. Refer to troubleshooting section in admin guide if it failed due to HostPlatformConfigFault Exception.
WARN [Thread-1582 (HornetQ-client-global-threads-235452xxx)] [com.emc.archway.service.vcenter.VCenterServiceBean] [XXXXX]  Fault String : com.vmware.vim25.PlatformConfigFault
If copy target LUNs are exposed to ESX, but they are not mounted (unmounted state) as datastores to ESX, then we may encounter this error.
Snaps of the datastore that are exposed to the ESX host but not mounted to the ESX host can be considered stale copies and can cause this error.
Unmask all the snapshots of the device in array from esxi host; so that none of the snapshots mask the ESXi host and then delete it. Rescan the bus on the ESX host to ensure they are removed.
For AppSync 4.0 and VCenter 6.5, the error seen in the CSV events will be:
2020-02-24 08:25:25.16,Mount copy,8be0c567-ff36-4cb1-afa1-a80cf1119727,VM_000086,USHOSTNAME,Mount of Datastore failed with error com.vmware.vim65.PlatformConfigFault@1e01a778 . Check the server.log for details. Refer to troubleshooting section in admin guide if it failed due to HostPlatformConfigFault Exception.