Hi Michael,
Please can you confirm what UD version you are running?
Can you also please reload the settings page and confirm that it still happens? (It was possible for this to happen on older versions before we introduced a check that the settings page belonged to the current version – e.g. people who update the plugin and don’t refresh the settings page before saving).
If it still recurs, after re-authenticating and then saving, please can you go to the ‘Advanced Tools’ tab, and in the ‘Site information’ section click on the ‘Show raw backup and file list’ link (scroll down a bit), and then within the window that appears, scroll down and find ‘updraft_dropbox’ and then paste for us the entry it shows? Then, after re-authenticating again, do the same, and send us the updated entry. In both cases, if it includes tokens in the output, change them, as these give access to your UD folder in Dropbox (e.g. replace half of the token bits with (snip)).
David