Ok, I followed all directions.
Downgraded to DSM 5.2 5592, after that I installed 5592 update 4.
My DS415+ is now loaded with DSM 5.2 5592 update 4.
After that I installed VirtualBox 5.0.8 from spk italia. Re-added my virtual machine (Server 2012 R2 x64).
Installed the VirtualBox tools and...