This issue disappear but another one appeared at VM restore: "ERROR: couldn't determine archive info from"
Our backup always worked on Proxmox 6.2 and the latest 5.
From now, dist-upgrade has run on Debian 10, everything is up-to-date; but still no luck.
Currently, it has to start with vzdump- then any of qemu lxc or openvz, then a VMID number.
The VMID requirement is dropped with aforementioned commit.