I searched Google and found that there is a registry setting to enable/disable it. However, mine was already set to 1, so it was not the cause. Then I realised I had disabled Aero Snapping in Settings -> System -> Multitasking -> Snap -> Arrange windows automatically by dragging to them sides or corners of the screen.
I had disabled it because the snapping was usually triggered unwantedly when I try to move the window. But it seems that dragging a maximised window is part of the Aero Snapping, and disabling it also turns off dragging maximised windows.
I turned Aero Snapping on, and dragging maximised windows worked again.
pelo criador da pergunta: Damn Vegetables