Camera is not moving

The camera/focus is not moving backwards. When you move the mouse, the camera/focus moves too. In my case it does forwards, left and right, but not backwards. What’s this, what can I do?

Maybe good to know: My laptop hasn’t enough hardware capacity due to the proper function of the game, for example: I couldn’t play a multiplayer game with more than 4 people. But: last year when I played it (had to reupload it) it worked fine, as long as I played with no more than 4 people overall.