Did you try to backup Hyper-V Virtual Machines with Windows Server backup and received Event ID 521 error on Hyper-V host? Backup started at '7.10.2008 23:21:14' failed as Volume Shadow copy operation failed for backup volumes with following error code...
Microsoft released RC1 version of Hyper-V. Description of the Hyper-V Release Candidate 1 update for Windows Server 2008 Update for Windows Server 2008 x64 Edition (KB950049) Windows Virtualization Team Blog