I've been able to upload files in the past to DropBox, but now it seems to be stuck in perpetual upload, with the pencil spinning. It's a fairly large file, but I've tried uploading it from my local folder and from the online folder, with no luck.
1. You can check the link to dropbox by going on to their website, logging in and looking in your settings > security > Devices. Sparkol should be listed. If you have any doubts about the link just remove it and create it again by following the 'upload to dropbox' process.
2. There may be an issue with the video file: Check validity by saving to desktop first (click the folder icon). If successful, you could then drop into dropbox from there.