The issue is the shared wall between your two cuboids. The inner volume must be completely open. This comes up all the time in 3D printing, as STL files are typically the accepted file format for this application. You can google around to find an STL repair service.
If you are creating the geometry yourself, see if you can't remove that shared face yourself.
I ran your stl through a service I've used in the past and found that, once repaired, your code works fine (attached, extension changed to txt)
