Backup Error: block device is in use by block job: backup

We're having a recurring issue with a couple of our VMs running backup jobs to PBS.

ERROR: Backup of VM 518 failed - VM 518 qmp command 'backup' failed - Node 'drive-sata0' is busy: block device is in use by block job: backup

There doesn't appear to be a lock on the affected VM, I've run 'qm unlock 518' and tried again with the same error. The only way to get past is to shutdown the VM (has to be full shutdown, restart doesn't work), and then the daily backup is successful for another week or so until it fails again.

All other VMs on the cluster in the backup job backup successfully (40-50, mix of Windows/Linux). I can't see any differences in setup between the VMs that work and this one that fails. It is a Windows 2019 server with the qemu-guest agent installed and working.

I've seen a few other posts about this in the forum, but it appears it resolved itself, or the OP didn't follow up with what they did to resolve.

Any ideas or things I could try? Anybody else encounter this?

Thanks.
 

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