I think I have found the problem: SDL2 for window creation.
Now I implemented my own window manager with contextual graphics bindings for Vulkan, OpenGL and DirectX (9/10/11/12) with a focus on threading and it seems that this is a good solution now.
Predefined libs/dubs like SDL2 or GLFW assume that the main handling exists on a static basis. I've spent hours trying to make it asynchronous, but it doesn't really work and only causes problems.
