In case you receive this kind of error:
'Microsoft Hyper-V VSS Writer' is in failed state, Writer Failure code: 0x800423f3
This error basically means Hyper-V has failed in some way.
The easiest next step is to visit http://backupchain.com and download VssDiag, a tool provided by BackupChain for its backup software.
Enter the time when the error occurred and it will go off and scan all relevant logs for you.
It should be clear from the error details VssDiag finds what the actual cause of error 0x800423f3 is.
BackupChain users may forward their logs using the Log View->Send to BackupChain Support. Ideally you should email us the VssDiag output, too
Hope this helps!