1. To check the existing configuration run the following PS command on any node of the cluster:
Get-ClusterResource "Virtual Machine Cluster WMI" | Get-ClusterParameter ConfigStoreRootPath
By default, “value” points to a cluster volume (e.g. C:\ClusterStorage\Volume1).Please note that this value can be set only once.
After it has been populated, you cannot change it anymore. This Microsoft article
provides more information.
2. Create a folder on that volume, and this folder will act as the meta files location. The folder should be available to all members of the cluster. (e.g. C:\Clusterstorage\Volume1\Hyper-V\Shared)
3. Specify the path to the newly created folder with the following commands:
$path = "C:\ClusterStorage\Volume1\Hyper-V\Shared"
Get-ClusterResource "Virtual Machine Cluster WMI" | Set-ClusterParameter -Name ConfigStoreRootPath -Value $path
The cluster now can be added to Veeam. If the cluster had already been added and the backup failed with the error in question, re-run the backup after setting the value.