[Gameplay] Unable to cast Lux's E.

Like in the video (and how he explains in german) I tried to relog, unbinding the key, switching smart cast and normal cast and nothing would allow me to cast it. The box is greyed out and the "You cannot cast this right now" text shows when you press the button. I believe it started when I lvled the ability up to 5. Pretty much rendered me useless and threw my promo game. Kind of frustrating.