Played around a little bit:
Copying FROM nas to PC without no Problems. Copied about 700 gig to pc.
I was curious and booted from a Debian Live USB Stick and mounted the NAS via NFS: Copying large files TO Nas without Problems.
In the end i formated the whole Nas Drive ( WD Red 2TB ) and resetted the nas to Factory defaults. But that did not fix the Problem.
I also changed the Router but that did not fix it.
BTW: I tested multiple Files, from 4gig to 22gig. Up to about 12gig it works fine. The 22gig File fails everytime. I can't count the times i pulled the power cord.
Maybe interesting: At the start of the transfer i have about 40mb/s transfer speed, the it cuts down to about 22mb/s and then it goes to zero and the 320 freezes totally.
I'm using Win 8.1 x64 and tested too with a Win 7 x64 PC, but that too failed.

In the end this Nas is not usable for me and i think i'll go for an other manufacturer, because i don't think D-LINK is able or willing to fix that in a reasonable time.