Thank you for looking deeper into it. Switching to Dx11 seemed to solve that problem. But now I get pretty much the same behavior just with another error message:
LowLevelFatalError [File:...\D3D11Util.cpp] [Line: 184]
Unreal Engine is exiting due to D3D device being lost. D3D device was not available to determine DXGI cause.
I just wanted to let you know. You don't have to look into it for my sake. I might check out the game a few months in the future again. Glad to hear that vsync was already planned :)