Auto Click after choosing a skill (cannot be deactivated)

When I press a key to use a skill (so Q, W, E, R and maybe number keys), the action is automatically performed! It is like making a mouse click directly after choosing the skill. This way I cannot target afterwards anymore and also not see the casting direction (blue arrow) before finally clicking. Nobody in the rounds I've played knew anything about this error. This behavior changed a few weeks ago. I have reinstalled Windows and of course the game since then.

How can I repair this and why did it happen on it's own in the first place??