Symptoms

The warnings are reported during backup (VSS-Aware for Hyper-V sets), for example:
The system cannot find the file specified. (\\192.xx.xx.xx\E$\Virtual Machines\Snapshots\2DBB9E48-FB8B-445A-8B8A-5258712C6D61.xml)
The system cannot find the file specified. (\\192.xx.xx.xx\E$\Virtual Hard Disks\Server_B833B6BD-A6B9-4272-9BC9-E90954C86FCA-AutoRecovery.avhdx) This is an issue addressed by Microsoft patches.
The operating system event log (System and Application) on the Hyper-V host and inside the backup source virtual machine provides the Windows related events.

Solution

1. Please ensure that the latest Microsoft updates for Windows 2012 R2 or the Windows version used are applied to the Hyper-V host as well as to the virtual machine backed up:
https://support.microsoft.com/en-us/kb/3135020.

2.Uncheck "Backup (volume checkpoint)" option (default is checked after the operating system is installed inside the virtual machine):
a) Start the Hyper-V Manager and connect to the Hyper-V host.
b) Right-click on the backup source virtual machine > Settings.
c) Go to Management > Integration services, deselect the option "Backup (volume checkpoint)".