Yes, I'm well aware, and I think that was it. I realized now - using my wife's laptop, that in the "Contents" section there has to be the order "author - title - image - custom text" in order to make the custom text movable from the "bottom margin" button. I'm not at home, but I'm quite sure that there I have left out the "image", and therefore nothing moved clicking on this button. If I do the same here, it's quite this behaviour as well.
Thanks a lot!
(By this way, I learned how to make a screenshot under Windows
)