Are you sure? It seems you did a snapshot on non-raw VM disk, which is OK. If you do a VM.snapshot, it will individually snapshot ALL the VM disk, including the raw ones. And this is when it breaks.
I would be very happy to get snapshots on raw drives, but it's not physically possible with SMAPIv1.
Yes, because on your other VMs, you aren't using a raw VDI 
RAW doesn't support snapshots, as explained in our doc: https://docs.xcp-ng.org/storage/#using-raw-format
I suppose the xoa backup hdd is a raw virtual drive, isn't it?
What kind of storage your XO VM (not XOA if it's from the source) is running?
