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Box is pretty bad at allowing uploads... They fail all the time which is pretty bad for a paid service. They have a new API coming out soon that will hopefully fix the issue.
In the meantime, you may want to look at Dropbox since we zero issues with uploading to them.
Bad answer, I know. But we have spent WAY too much time trying to get it to go. Box just doesn't handle large file uploads very well via their API.