Thanks for your reply.
Downloaded Shiki 2.0.3 and Lilu 1.1.4, put them in the E/C/k/O folder and restartet computer.
Unfortunately this did not resolve the problem.
Screen is connected via DVI, but a have also tested HDMI
(Sierra 10.12.5, GA-Z170M-D3H, i7 6700K, 32GB RAM, GeForce GTX950)