So the answer was: SnapDrive 7.1.3P3 is a buggy version. I had to go back to 7.1.2 to get this to work. Now with 7.1.4 it works even better because of better ONTAP 9 support.
Then I had a lot of other problems with my backups/snapshots but that has been solved now by running Application consistent snapshot on the VMs with the Intergration Services choice "Backup (Volume Checkpoint)" un-checked. After that I can check it again and the snapshots work great.
