avamar 7.1-avvss Error <5953>: Unable to allocate wide string of length 0.

Step 1-
Verify the number of VSS Providers listed.There should be only one provider as below-

1. Open cmd with "Run As" Administrator
2. At prompt type:vssadmin list providers

vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Provider name: 'Microsoft Software Shadow Copy provider 1.0'
   Provider type: System
   Provider Id: {b5946137-7b9f-4925-af80-51abd60b20d5}
   Version: 1.0.0.7

VSS backups are not supported if you have more than one VSS providers listed.

Step-2
1. Open cmd with “Run As” Administrator
2. At prompt type:Diskshadow
3. At prompt type:add volume c:
4. At prompt type:create

If you get the following:  COM call "lvssObject4->GetRootAndLogicalPrefixPaths" failed.
Then Avamar is not the cause of the VSS backup failure.See the following Microsoft TechNet KB article for the process:  http://technet.microsoft.com/en-us/library/cc772172.aspx

If you are able to create dishshadow, please proceed with following to now delete it from cmd prompt:

1. Open cmd with “Run As” Administrator
2. At prompt type:Delete shadows volume c:
3. type:exit

Step-3
1. Open cmd with “Run As” Administrator
2. At prompt type:msinfo32.exe to get the system information report
3. Also at prompt type:bcdedit.exe and see the output.

C:\Users\Administrateur>bcdedit.exe

Gestionnaire de démarrage Windows
---------------------------------
identificateur          {bootmgr}
device                  partition=\Device\HarddiskVolume2
path                    \EFI\Microsoft\Boot\bootmgfw.efi
description             Windows Boot Manager
locale                  fr-FR
inherit                 {globalsettings}
default                 {current}
resumeobject            {3a0f009f-cd65-11e2-b4af-3440b5d6bda6}
displayorder            {current}
toolsdisplayorder       {memdiag}
timeout                 30

Chargeur de démarrage Windows
-----------------------------
identificateur          {current}
device                  partition=C:
path                    \Windows\system32\winload.efi
description             Windows Server 2008 R2
locale                  fr-FR
inherit                 {bootloadersettings}
recoverysequence        {3a0f00a1-cd65-11e2-b4af-3440b5d6bda6}
recoveryenabled         Yes
osdevice                partition=C:
systemroot              \Windows
resumeobject            {3a0f009f-cd65-11e2-b4af-3440b5d6bda6}
nx                      OptOut

You will see the root cause of the issue which is EFI partition in result.
The EFI partition is not supported by the VSS backup with Avamar client 6.x.
However this will be supported in version 7 of Avamar.
As a work around you can backup the EFI manually using the Microsoft backup tool and let the Avamar backup the file created by the backup.

The use of "Extended Firmware Interface" (EFI) is not supported when performing Windows VSS System State backups."

For DR backup, please use Windows Server Backup perform system recovery (bare-metal recovery) backup to a share folder or a non-critical volume, then use Avamar backup to backup that share folder or volume.