VSS (Volume Shadow Copy Service) errors are not uncommon. VSS is a Windows component made by Microsoft and is the key to obtaining consistent and reliable live backups in Windows–there is no other way to achieve it.
The best starting point is to examine the Windows logs in the Windows Event Viewer, as shown here: http://backupchain.com/hyper-v-backup/Troubleshooting-First-Steps.html
In over 99% of all cases the errors will show up in the logs shown in the article link above.
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