This issue is caused when the Microsoft VSS service is attempting to do an application quiesced snapshot but is unable to read the disk UUID or Serial Number value for one or more virtual machine disks.
With ESX/ESXi 4.1 Update 1 and later, support for application quiesced snapshots for Windows 2008 and 2008 R2 Virtual machines was introduced. Application quiesced snapshot support required the new disk.enableUUID parameter to be added to the Virtual machine's .vmx configuration file to enable or disable the feature. When the parameter has been enabled, you need to reboot the virtual machine to ensure that the disk UUID/Serial Number is also populated as it is required for application snapshots.
The disk.enableUUID parameter did not exist on earlier versions of ESX/ESXi, therefore virtual machines created using those earlier versions will not have that parameter included in their .vmx configuration file. If the ESX/ESXi hosts are upgraded from an earlier version to ESX/ESXi 4.1, support for application quiesced snapshots will be available.
When a Virtual machine is migrated to a new datastore using Storage vMotion, the disk.enableUUID parameter is added to the Virtual machine's .vmx configuration file with a value of true if the parameter is not already present in the file. Before the migration, the effective value of disk.enableUUID is false (as the feature is not being used), thus the migration is changing the default value from false to true.
When a backup or snapshot attempts to quiesce the Virtual machine, it will see that application quiescing has been enabled (disk.enableUUID=true), but it will be unable to read the disk UUID/Serial Number as the virtual machine has not been rebooted since that parameter was added to the .vmx configuration file.