It seems that I was a little to fast there…
Having installed the plug-in to two other sites it seems that it is not working as it should.
The first backup is fine and OK.
Having authenticated my Dropbox, the plug-in created a subfolder with the site-name and there it stores the backup. OK fine!
The second and third.
Having authenticated my Dropbox, the plug-in creates nothing. No sub-folder and the backup-proces never starts.
The log says: “Send request to create backup” as the last thing – then it stumbles and nothing happens.
I have tried to log out of Dropbox and re-authenticated but unfortunately I get the same result.