Bumping this thread as I appear to be running into the same issue.
Error logs:
INFO: filesystem type on dumpdir is 'zfs' -using /var/tmp/vzdumptmp991849_103 for temporary files
INFO: Starting Backup of VM 103 (lxc)
INFO: Backup started at 2022-01-24 04:00:55
INFO: status = running
INFO: CT...