Have you tried uploading when you're connected via WiFi?
Some Google users (Picasa) had this problem and one cause was corrupted files. Others found a solution in making sure they checked the box to Limit Local Bandwidth. It appears hosting sites can reject files being uploaded if they are sent too fast. Crazy, but it's prevalent. I don't know if Box has a setting somewhere (maybe on your account if you access it via pc?)
Perhaps you should head over to the Box support forums and ask them about it. They have been accused of throttling free users to preserve bandwidth for paid customers. https://support.box.com/entries/2215...lmost-unusable