Sometimes a backup or snapshot process is killed blocking any further backup. An LXC (or QM) may be left in a locked state and can not be unlocked. An IO error like this is thrown:
$ pct unlock $VMID
unable to open file '/etc/pve/nodes/$NODEID/lxc/$VMID.conf.tmp.32442' - Input/output error
Neither file nor process exist. The file can also not just be created, because it’s on a protected fuse mount. What usually helps is restarting the PVE-Cluster service (even on a single cluster node):
$ systemctl restart pve-cluster
$ pct unlock $VMID