NetApp hardware VSS provider limitation

Hyper-V Backup/Replication Job utilizing NetApp Data ONTAP VSS Hardware Provider reports warning or error message:
"Another shadow copy creation is already in progress. Please wait a few moments and try again"
 

NetApp hardware VSS provider doesn't allow concurrent snapshot creation within cluster, even when snapshots are initiated on different hosts and for different CSVs.
With NetApp hardware VSS provider you can have only one snapshot creation running within cluster at once - there is no concurrency.

Microsoft software VSS provider (Microsoft CSV shadow copy provider) has no such limitation.

You can switch to Microsoft software VSS provider:
https://helpcenter.veeam.com/backup/hyperv/hv_server_volumes.html#volume

Or disable concurrent shadow copy snapshot creation within cluster for all providers:
Create HyperVRestrictConcurrentSnapshotCreation (DWORD) registry value under HKLM\SOFTWARE\Veeam\Veeam Backup and Replication key on the Veeam backup server.
And set it to 1.
Then stop all Jobs and restart Veeam Backup service.