This error suggest that the OAuth redirect URIs in your own Dropbox App configuration doesn't match the redirect url required by the plugin. the
- Go to the tab Advanced ->API Application -> Use Custom App on the plugin options page.
- Copy the OAuth 2.0 Redirect URI value.
- Go to your Dropbox App console.
- Configure your App, and paste the copied value in the Redirect URIs field in the settings section of your app.
- Save the configuration.
The OAuth redirect url has changed in the past, so check the current redirect url as shown on the plugin settings page: tab Advanced ->API Application -> Use Custom App.