Ion Maiden No mouse in-game


I just bought and downloaded Ion Maiden, but I get no mouse in game. In the menu I see a mouse pointer move if I move the mouse, but as soon as I start a game, the mouse no longer does anything. No mouse-aiming, no firing, nothing. I’ve checked the control settings and I don’t see an option to turn on mouse aiming, so it seems to me it should be on by default, but I just get no mouse.

Anyone any idea what I have to do?