It looks like this plant has some kind of problem, although I don't know what it is because all my conditions are correct according to the information here and the plant should be fine. I used fresh soil (top layer of BioBizz Allmix, approx. 3 liters and underneath approx. 14 liters of BioBizz AllMix) and watering has not caused any problems so far. I'll let it run like this now, before I possibly break something with wrong countermeasures.