When using Veeam to back up your VMs, and you get the following error on one or more VMs:

Error: VSSControl: -2147212529 Backup job failed.
Discovery phase failed.
Cannot add volumes to the snapshot set.
Cannot add a volume to the snapshot set. Volume name: [\\?\Volume{6c169bc7-d7bf-11e0-b074-806e6f6e6963}\].
Cannot add volume to the set of volumes that should be shadowed.
VSS error: VSS_E_UNEXPECTED_PROVIDER_ERROR. Code:0x8004230f

According to KB1785, this is because of a third party VSS provider, present on the VM. If more than the default software provider is installed, another third party backup software suite may have enabled their provider on the VM, causing the initiation of a VSS backup to fail, resulting in Veeam no longer being able to create consistent backups of the affected VM.

To resolve, the third party VSS provider will have to be removed. How to do this, depends on the installed third party software. Consult their documentation for more information on removal. It may be possible to only remove the VSS provider while keeping the rest of the third party software intact.