Hi @Andi_M8 , I have 3 possible issues for you.
Possibility 1. Magnesium deficiency
Possibility 2. Potassium deficiency
Possibility 3. pH problem
I don’t know if and if so how much magnesium you’re giving, but a pinch of Epsom salt should prevent it from getting worse if that turns out to be the problem.
Potassium deficiency can be caused by too much nitrogen, but I don’t know your fertilizer and feeding schedule, check to see if that applies so you can counteract it.
For a pH problem in the substrate, simply test it; if it’s fine, you can rule that out.
I hope I was able to help you out. Best regards