BackupAssist and Hyper-V – where’s that error coming from?

We've seen a noticeable increase over the past few months in the number of customers performing Hyper-V backups using BackupAssist, so I'm hoping some of you will find it useful to hear about an issue we helped resolve recently. The customer was running BackupAssist on their Hyper-V host system and had initially been performing a backup of the system state and some data which was working fine, when they added the Hyper-V guest machines though, it produced this error each time: ERROR - A Volume Shadow Copy Service operation error has occurred