I have reproduced this issue on Windows 10 Enterprise Insider Preview Build 11082.
The Hyper-V VSS Writer is not present when running "vssadmin list writers"
Restarting the Hyper-V Management Service had no effect - the backup still failed, and the Hyper-V VSS Writer was still missing.
Machine Name: FFL-MDT01
Complete Server Logs:http://1drv.ms/1Ps6ErK
Support Case ID: 01250222
To clarify, the Hyper-V Host is Windows 10 Enterprise Insider Preview Build 11082, the Guest VMs to be backed up are running Windows 2012 R2 Datacenter.