Ok, can you do two tests for me?

First, scale down the cubemap to half its size and try whether this solves the problem. It is known that 512x512 cubemaps won't work on most systems, including some that otherwise support 4096 textures.

Second, can you try your level on your PC with 6.31 and 6.40 and post two screenshots to compare? I'd like to see how the shader is supposed to look.