Symptoms

The warning might be reported during backup (VMware VADP sets).

Cause

The option "Attempt to use CBT" has been selected in backup set's properties, the backup source vm contains thin or thick lazy vmdks and the CBT is broken, details are available in VMware KB 2035976.

Solution

Remove any snapshots available for the backup source vm and then disable and re-enable the CBT.
To disable and re-enable the CBT using the vSphere Client:
1. Shut down the virtual machine.
2. Remove any existing virtual machine snapshots.
3. Right click the VM, click "Edit settings", find the "Options" tab and click "Configuration Parameters"
4. Set the "ctkEnabled" value to false
5. Set the "scsi0:x.ctkEnabled" value to false for each disk of the VM in question
6. Remove or rename any files ending with the *-ctk.vmdk file extension in the virtual machine directory.
7. Power on the virtual machine.
8. Power off the VM again.

Then enable back CBT: 

9. Right click the VM, click "Edit settings", find the "Options" tab and click "Configuration Parameters"
10. Set the "ctkEnabled" value to true
11. Set the "scsi0:x.ctkEnabled" value to true for each disk of the VM in question
12. Power on the VM
13. In the home directory of the virtual machine, verify that each hard disk contains a vmname-ctk.vmdk file.
14. Run the backup.