Was könnte es sein ?

If the pH level is normal and no fertilizers have been used, then it is most likely an NPK deficiency. Start giving fertilizers according to the instructions and observe the results for 3-4 days. Ideally, measure ppm/ec at the output – the plant has already consumed all the nutrients from the soil in 4 weeks.

it's a mobile nutes at the root of this, since it;s starting mostly at the bottm and moving upward. It's not super clear as to what is most likely the cause here. You would definitely need to crossreference some guesstimation of what ratios of nutes have been applied and at what concentration.. then you could better guess.. .or, wait for it to progress a bit more and hope the symptoms clearly present a specific cause -- not always the case, but a good chance of it. I want to say potassium deficiency, but i don't think it quite fits. Mg deficiency is possible if the plants are old enouhg (it takes 4-6 weeks from inception of a mg deficiency before you see visible symptoms on leaves). The burning of those tips in that fashion could be P or K tox. this would be accomanied by pale new growth with thinner blades and possibly locking out things like Mg which might explain the slight mesh of symptoms seen. I do see some hints of yellowish new growth -- even if it fills in shortly after.