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

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http://serverfault.com – 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 (HowTos)