Thank you Martin.
Seems I cant edit my original post (yet maybe, because I am a forum noob), so here the change-location:
In the NetApp ONTAP System Manager in the Storage>Volumes-Settings for my volume, I found the “Snapshot Reserve %”-Value. Standard seems to be 5%. Gladly in my case the snapshots takes most of the storage of this volume and I could increase it massively to 20%.
Hurray for checkmk.