Hello Jeroen,
I think we're going to need more info to be able to help out. Firstly, I've just tried moving a task to the root of my outline (in Windows, by just dragging the task with my mouse) without any problems. So, the MLO software isn't the problem.
Let's try creating a task in the root -
If you select a task that's already in the root and press "Ins" (assuming you haven't changed the hotkey), does it create a new sibling task, also in the root folder?
Alternative method - try right-clicking on a task in the root and select "New Task" - that should also create a sibling in the root.
Moving a task into the root folder -
Try selecting a task and then dragging it down your list until the insertion line is where you want it - does that work?
Try the hotkey for promoting a task up the outline until it's at root level (on my laptop that's Shift+Alt+left arrow - can't remember whether that's a fixed hotkey or custom shortcut key).
If those options don't work in your outline, try creating a new outline and create a few tasks and subtasks in that, to see if your original outline has some kind of corruption (which is a very, very rare occurrence).
If you still can't create tasks in the root, you may have to try reinstalling MLO (or use the portable app version of MLO, like I do).
Hope that helps you track the problem down,
Stéphane