Box Sync 4.0 provides detailed user notifications if there an issue syncing a file or folder.
This rare issue may occur when Box Sync is resolving a file name conflict. If there is already a file in the sync destination location with the same name, you will see this error.
All devices with Box Sync installed.
Check to see if there is already another file with the same name in the sync location. If so, rename the file and try to resync it.