Thanks for checking in @Falco. It did fail - the info sent to the logs was very clear and indicated the wrong key.
I had the same problem as @ckshen. The language below the key input field says
Your Dropbox API key created on https://www.dropbox.com/developers/apps/create
Of course, useless without the
secret, but I followed the language a bit too literally probably.
Perhaps it could say:
Your Dropbox Access Token generated at https://www.dropbox.com/developers/apps/create
Just to prevent fools like us returning
Now I'll just wait to see how the next backup goes.