There is likely a problem with the header of the file. Do you have the Cube3Editor.exe? You can look at your files using this app by our own BuddyBu.

I still have not gotten Simplify3D so I cannot directly help you. Hopefully someone with more experience will chime in.

I think it was CubePro 1.87 was the latest version of the file you can read after decoding. Decoders for more recent app versions are not available. This will let you make some native files that you can compare in the cube3editor.

This is where I'd have you go to diagnose the issue. Post your findings and we can see what is obviously missing or included.