Was kann ich machen?

The flowering is going well, but it could be an exaggeration of the nutrients. However, the pistils look good. Are you using the nutrients with their respective chart?

IF it is Ph, you should flush with Ph-balanced water to reestablish the correct values. But this is a big IF without info (e.g. a diary).

What can I do about it? Water with distilled water or flush first?

Not a lot of information, but it seems some nutrient block. Did you check your Ph? Last run I experienced Ph issues, with consequent nutrient block, which look like in your pictures. With wrong Ph the plant is not able to get nutrients, and try to absorb them from the reserves in the leaves, which turn yellow, brown and then die. If you didn't I suggest to buy a Ph pen and make sure to fit the range (I go usually around 6-6.3, which is also good for Calcium)

The yellow leaves bother me

Would you give us some indication of what concerns you?