Windows Home Server 2011 and Backing up UEFI-based computers

After upgrading my computers from Windows 7 to 8 my backups, made using Windows Home Server 2011 (which I run as a virtual machine for the sole purpose of backups), did not work. I got this error on the client machines:

Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface.  hr = 0x80070005, Access is denied.
. This is often caused by incorrect security settings in either the writer or requestor process.

Operation:
   Gathering Writer Data

Context:
   Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Name: System Writer
   Writer Instance ID: {bc66abce-f123-4f20-9121-8ebe21c3134f}

It turned out this error message is rather misleading. What was really the problem was that Windows Home Server 2011 does not support client backup on Unified Extensible Firmware Interface (UEFI)-based computers that contain a GUID partition table (GPT)-formatted disk. But luckily, there is a hotfix from Microsoft: http://support.microsoft.com/kb/2781272/en-us

After applying this hotfix, reinstalling WHS client software and extending the virtual disk for WHS (it was full), the backups started working.