I've got a bit of a problem. I typed in my password and it seems to be impossible to get back to the dialog. And removing or even purging the application doesn't remove the stored username/password settings. The error screen suggests I click the back button, but there is no back button on the first page, as you can see from the screenshots in the original post.

Never mind. I had to remove the account from "accounts". That part is missing an icon btw.