Any backup process can be interrupted before it is finished due to various issues, such as backup storage node failure, loss of network connectivity, very large virtual disk images, and so on. Prior to starting the actual backup process, Proxmox puts a global lock on the VM so that multiple backup tasks cannot be run on the same node. If the backup is not finished successfully, this lock sometimes remains in place and is not automatically removed. If we try to start/stop the VM, we may see an error message that informs us that the VM is locked.

In such cases, we need to manually unlock the VM to resume the normal operation. The unlocking cannot be done from the Proxmox GUI, but only through the CLI. The command will need to be run in the node where the VM is. The following command will unlock a locked VM in a Proxmox node:

    # qm unlock <vm_id>