Oh dear, this is what I feared when Vates added the registration limits. Now, if there is a problem with XOA then we can't install a new XOA in order to access backups... I suppose it is possible to register using a fake email. 
EDIT: I was lucky. Had a backup exported as OVA and could re-import using xcp-ng center...
Still I think this is critical. There should be a ready-made appliance that allows restoring of backups without internet access.