Cubemaps not working, Background is in lower resolution

Hello,

so I made a completely new background using the Background Example from the workshop.
My planets and custom lens flares are working, but my background is not as sharp as it should be.
It is 4096x2056 pixels big, like the Homeworld backgrounds.

And the cubemaps are not working, the reflections are just plain black.
I have _pos* and _neg* .tga files in the folder with it.