Windows Server Backup fails to backup Hyper-V VM with "Access is denied"

I'm trying to use Windows Server Backup on my Windows Server 2012 box to backup my Hyper-V VMs. I created a backup job but each job ends with some "Access is denied" errors.

One of my VMs (Linux Server) is backed up properly. All others (one Windows 8, one Linux) are not (or at least it seems that way from the looks of the log file below).

How can I solve this problem?

Here's the log I'm getting:

Error in backup of D:\ during read: Error [0x80070005] Access is denied.
Application backup
Writer Id: {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}
   Component: C435964E-C07A-4958-BA73-A04C6583280F
   Caption     : Backup Using Saved State\Alter Server
   Logical Path: 
   Error           : 8078010E
   Error Message   : Copy of the files failed.

   Detailed Error  : 80070005
   Detailed Error Message : (null)


Writer Id: {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}
   Component: E780F138-9676-42FB-821C-4561B9B263DC
   Caption     : Backup Using Child Partition Snapshot\Windows 8
   Logical Path: 
   Error           : 8078010E
   Error Message   : Copy of the files failed.

   Detailed Error  : 80070005
   Detailed Error Message : (null)


Writer Id: {66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}
   Component: Host Component
   Caption     : Host Component
   Logical Path: 
   Error           : 8078010E
   Error Message   : Copy of the files failed.

   Detailed Error  : 80070005
   Detailed Error Message : (null)

It seems that Windows Server Backup can't handle Hyper-V VMs on ReFS file systems. I solved the problem by changing the file system to NTFS, and now everything works fine.