Box Sync 4.0 provides detailed user notifications if there an issue syncing a file or folder.
When Sync tries to delete a folder and the folder is not empty, you may encounter this error. For example, you will see this error when you add a file to a folder on your computer while it is offline and you unsync the folder on Box before allowing Box Sync to make the changes.
All devices with Box Sync installed.
Move the remaining files in the folder to another location and then resync the folder.