On a Windows Server 2012 R2 VM protected by Veeam Backup & Replication, Veeam’s application aware processing was failing with the error below.

Retrying snapshot creation attempt (Could not create backup checkpoint for virtual machine ‘%VMNAME%’: The volume shadow copy provider is not registered in the system. (0x80042304). (Virtual machine ID 464FEFAA-0EF5-4E9F-98BF-D76EBE72717B))
Failed to create snapshot (Dell EqualLogic VSS HW Provider) (mode: Veeam application-aware processing with failover) Details: Writer ‘Microsoft Hyper-V VSS Writer’ is failed at ‘VSS_WS_FAILED_AT_PREPARE_SNAPSHOT’. The writer experienced a non-transient error. If the backup process is retried, the error is likely to reoccur. –tr:Failed to verify writers state. –tr:Failed to perform pre-backup tasks.
Retrying snapshot creation attempt (Writer ‘Microsoft Hyper-V VSS Writer’ is failed at ‘VSS_WS_FAILED_AT_PREPARE_SNAPSHOT’. The writer experienced a non-transient error. If the backup process is retried, the error is likely to reoccur. –tr:Failed to verify writers state. –tr:Failed to perform pre-backup tasks.)
Guest processing skipped (check guest OS VSS state and hypervisor integration components version)

The VM is also protected by Azure Site Recovery/Hyper-V Replica and the virtual host running the VM logged a “The volume shadow copy provider is not registered in the system” error each time an application consistent recovery point was attempted.

I checked the registry key HKLM\SYSTEM\CurrentControlSet\Services\VSS\Providers and noticed one of the providers ({74600e39-7dc5-4567-a03b-f091d6c7b092}) was missing a CLSID key.

I found another VM running Windows Server 2012 R2 and navigated to the same registry key.  In this case the CLSID key was populated.  I copied the CLSID key from the working system and applied to the VM with the issue.

Next I restarted the Hyper-V Volume Shadow Copy Requestor on the VM and found that the VSS error was resolved.