first of all I want to say that this a is really nice idea for an plugin. It will helps a lot. And the upcomming "Cronjobs" will be a killer feature.
I set it up and run on PHP 5.2.11. The files are created on my server and stored under wp-content. If I have understood everything correctly, it should be loaded from there to the drop box and then be deleted. Both does not work.
I've used my drop box and e-mail and password were correct. Doubble and triple checked. Then I set up a new Dropbox - even for security reasons I wanted to do this. But even with this drop box will not do.
Is this a known problem? What's the problem?