I had the same problem for weeks. No solution described on the net helped me. Neither registry changing, nor Windows-Patch from Microsoft -sites, nothing. My really solution was activating the Windows partition
(by me this is the drive C
via partition manager (Start menu -> search "Create format and hard disk partitions" -> ENTER -> Disk Manager comes up -> right click on the partition which has booting Windows system -> "Mark partition as Active" -> restart the system -> backup worked successfull again). Yes, the strange thing was here, that the system partition was not active !
Only this change helped me. Try out! It should help you!