Ok Kees, I'm now confused.
The original question related to the 'mycourses' menu and the title of the post is 'I cannot scroll in the overlay-menus'
And yet you are now discussing the site administration block which does have a scroll bar - red thing on the right in your screen shot - and yet you say "The "mycourses" is not the problem, scrolling is" - and yet in that menu which you now claim you cannot scroll there is a scroll bar.
In reading your words you are also implying that when the site administration menu is there and you've not expanded any of the options and yet there is more in the menu then you cannot scroll on the page? But as the menu is docked and you think you can't scroll, you can as there is a scrill bar! I've annotated your screen shot to illustrate this.
Whenever I shrink my screen and there are too many items to fit on the screen in that menu the scroll bar always appears. Therefore "If the "settingmenu" is to big for one screen (there are more options than the screen can show), then I can not scroll down to get the latest menu-options." is wrong.