Sounds like there's some sort of processing that's happening on the server side that blocks the file from being accessible with Insert From URL. Unfortunately you can't control either the insert from URL behavior on filemaker's side or the server side behavior from box.com.
What you might be up against is using Box's API to retrieve the file.
It appears that box.com's view API supports cURL, so that might be a good place to start.
The free baseelements plugin supports cURL.