Was hat sie?

Hey, that looks like a light nutrient burn to me. The brown tips usually give it away — that's where salt builds up that the plant can't get rid of. It happens quickly if you over-fertilize a bit. The Plagron Lightmix is already pre-fertilized soil, so I would suggest running the Biobizz manufacturer schedule a bit reduced. Fertilizer manufacturers often assume completely "clean" soil. If you lower the nutrient dosage a bit now, the new leaves should come back nicely. The affected leaf won't recover, but that's not a big deal. And yes — week 4 fits well with the picture. 😊 I think starting from week 6 you can begin to ramp up the feeding again, and by week 7/8 you can follow the manufacturer schedule again.