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I have an error from a Veeam Hyper-V backup of a Virtual Machine in a Hyper-V host:

Unable to create snapshot (Microsoft CSV Shadow Copy Provider) (mode: Veeam application-aware processing). Details: Writer 'Microsoft Hyper-V VSS Writer' is failed at 'VSS_WS_FAILED_AT_POST_SNAPSHOT'.
The writer experienced a transient error. If the backup process is retried,
the error may not reoccur.
--tr:Failed to verify writers state.
--tr:Failed to create VSS snapshot.
--tr:Failed to perform pre-backup tasks.

I cannot understand if this is a host error, a guest error or Veeam error.
I tried to reboot the VM but I got the same error. I did not try to reboot the host, but I tried to change the host (this VM is inside a failover cluster of two hosts) and with this change the backup works again.

During the backup failure I ran this command in the host:

 vssadmin list writers

The output was:

>    Writer name: 'Microsoft Hyper-V VSS Writer'

>        Writer Id: {66841cd4-6ded-4f4b-8f17-fd23f8ddc3de}

>        Writer Instance Id: {f50fe40b-27d6-4a98-8713-e5a1dce54c97}

>        State: [1] Frozen

After the VM migrates to another host, the writer state changes back to "Stable".

What can freeze the VSS writers?

There is a 10103 event in the Host:

Failed to revert to VSS snapshot on the virtual hard disk '\?Volume{29e16157-002f-4cb4-af58-c95acbd54be9}' of the virtual machine 'srvshare1'.

Is the virtual machine a domain controller?

Have you tried below from http://www.veeam.com/kb1697

vssadmin list writers

The results will appear as:

Writer name: 'NTDS'
Writer Id: {b2014c9e-8711-4c5c-a5a9-3cf384484757}
Writer Instance Id: {ee24b741-eaf7-4663-8f95-b92ae8c5e164}
State: [1] Stable
Last error: No error

If not listed as stable reboot the DC.

──────────────────────────────────────────────────────────
Verify that Automatic mounting of new volumes enabled.
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From a Run command (Win+R) execute the command diskpart.

From within Diskpart run the following command.

automount

If the results do not show “Automatic mounting of new volumes enabled.” Run the following command:

automount enable

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Verify that there are no .bak keys in the ProfileList within the Registry.
──────────────────────────────────────────────────────────
From a Run command (Win+R) execute the command regedit.

Within the registry navigate to:

HKLMSoftwareMicrosoftWindows NTCurrentVersionProfileList

There will be a list of Keys, you must remove any ending in .bak