Can't lock vz with proxmox

I would like to backup my virtual servers to move them away. And when i have started the backup process then i only see this every time:

Feb 09 08:40:34 INFO: Starting Backup of VM 101 (openvz)
Feb 09 08:40:34 INFO: CTID 101 exist mounted running
Feb 09 08:40:34 INFO: status = running
Feb 09 08:41:34 ERROR: Backup of VM 101 failed - can't lock VM 101

If you need any more details then i will give them for you! Thanks in advance for your help!

Solution 1:

Lock files are stored here in /var/lib/vz/lock/. Any chance this directory is not writable/missing ?

Solution 2:

Try to unlock the VM with this:
qm unlock VMID