Okay, our problem is partially solved.
The snapshot can be mounted now. The problem seemed to be that "create volume-snapshot for multiple vms..." has been enabled by default, but never seemed to work. As if it has not been enabled.
After disabling this option and re-enableing int again our snapshot(s) can be mounted on the off-host proxy.
BUT ... now we have the problem that volume snapshot is created, mounted, used and unmounted by veeam at the end of the backup job, but the created netapp snapshot never gets removed.
I guess I will have to contact NetApp for a solution.