I have no clue what a "normal soil mix for balcony flowers" is but looking at those pics...its either abnormal ph (should be 6.2-6.8 for soil runoff and for the water you use to water with..."around 7".. is a little less specific than ideal) or whatever is in your soil mix is burning your plants. I'm guess ph problem but you'll need something better to measure your ph with to be sure.

PS Not overwatering are you? That will make ph problems (if you have them) worse.