After a quick test I have the impression the boolean is just a bit too complex for SketchUp. It happens (sometimes) and you have to manually clean it up until its a boolean again. Maybe if you try to split the objects in half and mirror them after the boolean it might work.
To clean it up, I would add a sectionplane (and hide the plane) so you can more easily select interior faces that need to be removed. Just continue until the group 'says' solid again.


