@Butzemann, try adding phosphorus and manganese. How much calmag are you giving? In my opinion, it can only be phosphorus, calmag, manganese, or over-fertilization/lockout from the soil. I would just give some of that; I don't think it can hurt. Where did it start, at the bottom or the top?
Hey, can you write to me? I'm new here and have no clue yet. I have a question regarding my report about the yellow leaves that you commented on. Should I stop giving nutrients for now? I actually followed the guidelines.