I have reset plugin 2 times, the second one first deauthorizing the App at dropbox. I both times I changed the temp folder to the default recommended by your plugin (it was defaulting to something else because of the theme I am using...)
I even changed permissions of the folder to 777, to play it safe...
I both occasions, thru serveral attempts I got either of both of this messages:
Error!!! Unable to connect to ssl://api-content.dropbox.com:443. Error: Connection timed out
Error!!! Forbidden. This could mean a bad OAuth request, or a file or folder already existing at the target location.