Ok i solved it for my. Although its a little bit awkward, i would like to share my "solution". Maybe there is someone else who did the same mistake.
The solution is quite simple, it had nothing to do with PoE2 oder the unity engine. It was just my GPU, which was in the wrong PCI-E slot. After switching the GPU in the other slot, now everything runs smoothly.
If you want to test this, simply download GPU-Z and check in the "Bus Interface" field, if your GPU runs right. If you point your mouse some time over the field you get some aditional information.
Wrong would be "PCIe x 16 x 2.0 @ x 1 1.1"
Right could be "PCIe x 16 x 2.0 @ x16 2.0"
Good luck!