Chefmichel
Your picture fell off of the post.
That is more than likly why it is not answerd.
I saw the picture and I think it is some kind of bug.
if this helps.
I'm way sorry for no reply in like 4 days.
Worth

The picture in the middle is just too blurry to tell anything.

As far as the last one, did you spray anything? Fungicide, etc. Or it might be related to nutrient solution or PH?? I just don't know anything about hydroponic gardening.

This is an interesting site: Click on one of the topics on the left and it brings up articles on that topic. Maybe check out nutrient def, etc.

Here is a link that might be useful: Grodan-greenhouse growing

Thank you very much Farkee. Grodan gives a lot of information
on deseases and other topics.
Meanwhile, i cut out the sick leaves, and the plant is OK.
There are no bugs, i checked very carefully.
It must have been something with the nutrient as i grow hydroponically. As a test, i planted in soil the same day. Hydroponically grown plants are growing much better than the same test group i planted in soil.