UE4 shaders/materials broken when starting in steam

The entire Scene is black, except the skybox, but only when I run the game via steam. Running without steam works fine. Is there anything in the logs I can look for? It says “Failed to load Shared Shader Library” even if I run it without steam, so that’s not the issue I assume.

I was able to figure out something more:
I’ve 2 levels, let’s call them A and B.
A is using a post-processing volume, B is not.
A is completely black as I mentioned, however, in B there’s colours, but shadows are missing.
Only when I run it via steam, everything works as expected without.

apparently it was the “experimental” opengl/vulkan support for windows. switching to d3d solved it.