Kann mir jemand sagen was mit ihnen los ist?

Magnesium deficiency. Magnesium is being blocked from your plant by too much calcium in your tapwater. Switch to distilled water, or RO water. Or you can add Epsom salt (Bittersalz) to your tapwater to correct calcium to magnesium ratio. Add 3 pinches of epsom salt per 2 liters of tapwater... half a teaspoon. Ask for it at the apotheke. Make sure you water pH value is between 6 and 7. You can get a pH test kit for water in stores with an aquarium supply section. You will likely need to adjust it down. A citric acid type like Biobizz Down is good for soil. And never buy anything from fastbuds anymore. They are the most low quality seeds available that only sell because of hype. Get better quality seeds. This is the fastbuds website by the way. So finding a better platform/forum on a different website is highly recommended.