Hi,
Alternative way that works:
1- keep the plugin activated;
2- in another window or tab, keep Dropbox account logged in;
3- Update the plugin to the newer version;
4- Go to wordpress dashboard sidebar WPB2D/Backup Settings;
5- Click “authorize” button, you will be directed to the dropbox and get the known error;
6- back to the wordpress tab and try again by clicking “Continue”, then you’ll see the error again, it also releases the authorization button again.
7- click “authorize” and you’ll be redirected to dropbox authorization page again;
8- click allow button;
9- back to your wordpress tab and click “Continue”, than you’ll see the plugin working again.
Hope it works for you too.