SOLVED it.
My omv-export starts with: /export/NAS-DRIVE ...
From ubuntu I can mount it with mount -o .... <omv-nfs-server-ip>:/NAS-DRIVE /mnt/<my_mount_point>
Well this is good for nfs_v4 on both sides!
Coming from the old world... nfs_v3/win10
you a have to spent the complete path given in the export file at your omv-server.
mount -o anon \\<omv-nfs-server-ip>\export\NAS-DRIVE <drive_letter>
Took me hours to figure that out. Hope that helps anybody.