We have the same Issue with NetApp ONTAP 9.0GA! The backups can't transfer snapshot to snapvault because it says the relationship cant be found for the snapvault. @Joje: Have you an open Support Ticket for this Issue?
by Joje » Mon Jun 19, 2017 9:27 am people like this post
Yes, the customer logged a support ticket regarding this and veeam says its a bug in 9.5U2 with XDP (mirror-vault). I haven't been at the customers site since then but they have a patch for this issue if you raise a ticket with them.