Restore failed of public cloud instances

Hello,
i have a couple of instances on OVH Public Cloud.
I did some backups of these according the documentation, no errors, but the size is 0 , somehow strange to me
I followed the instruction for the restore, choose my backup (different ones) and run every time into an error "an error has occured.......unable to rebuild with a diffrent image for a volume-backed server"


thx for any help