Hi, @peopleinside and @cappieq!
Please make sure to configure Dropbox API keys by following the steps in this user guide section. I am also posting it here quoted below:
Visit Dropbox Apps page and log in to create API keys and add them to corresponding settings of this page. If you don’t have any apps, press Create Apps button from Dropbox App Console.
Select Dropbox API option, then choose the access type, Sandbox App or Full Dropbox App. Name your app, then hit Create App button. You will be redirected to a dashboard page, where the App key and App secret will be provided.
Note, that Full Dropbox App and Sandbox App have different API keys. Make sure to add yours to the correct options of Backup WD plugin.
Afterwards, edit your backup job and configure the settings of Dropbox tab following the steps described in this section.
Let us know in case you have questions.
Thanks! Have a great day, guys!