Cubemap Reflection from Texture2D?

Hello, i ran into an issue again wich is bugging me long time now.

Here they tell its just ReflectionVector and a Mask to obtain the right TexCoords for “cubemapping” a Texture2d

but it is not. Thats my Texture:

Thats the Result:

One could think thats just 90° swapped and some multiplications, but its really not. so whats wrong here?
I can not use the cupemaps, so i have to stay with texture2d in this case.

thanks a lot.