"Overview" is what Plasma shows you when you move your mouse to the top left. It is supposed to show you your virtual desktops and allow you to move between them, but in fact if you have more than 2, it doesn't show any. You have to remove them again to two for you to be able to see the virtual desktops.
Actually this only happens if you try to put multiple virtual desktops in "one" line. That's the issue
*** This bug has been marked as a duplicate of bug 475077 ***