i haven't found any similar Thread here, now i believe i'm doing some obvious mistake..
When i create a Backup job in Veeam Agent for Windows, i can't select my D:\ Drive as Backup source. When i choose to Backup the entire Computer, my C:\ Drive and System Partition are Backed up, D:\ isn't.
When i choose the Volume Level Backup, the D:\ Drive isn't showing up either, even if i select to show hidden drives, here a Screenshot:

System specifications:
Win Server 2016 Standard (Hyper-V Host)
128 GB RAM
2x Xeon E5-2643v4 6 Core
Missing Drive:
RAID 10 Volume (4x Intel P3700 800GB NVMe SSD, RAID created in Intel RSTe Software), 128KB Stripe Size
Basic MBR NTFS Volume
Contains the .vhd Files for Hyper-V
I've re-created the RAID Volume and formatted it already, so i don't think a faulty filesystem is the problem here.
I can't see Errors in the Logfiles either, where should i even look there?