Hohe Luftfeuchtigkeit im Gewächshaus

Your plants are in veg and high RH is ok for them. its not ideal but it won't harm them. The thing you want to avoid is higher RH in flower like 70%+ as it can help molds grow and they then rot the the buds on the plant. Keeping them out of direct rain fall also helps so the buds are not wet + high RH. High RH can also cause something called WPM, and its a bitch, its a fungus that grows on the cannabis leaves. Its hard to kill and some products your spray on the leaves help a bit, like 30% but the fact your getting the plant wet is a bigger issue and I avoid them. I would not use cat litter in them, that stuff is clay or salt crystals with a bunch of other crap in it, and as soon as its saturated it will become useless and cause issues in the medium if you sprinkle it into the pots high EC burning or turning it into mud. There is no easy way to lower the RH other then using a dehumidifier, but given your outdoors this will have no effect, its mostly for indoor grows. If you always have high RH growing sativia like stains that are more airy buds is good as it allows proper airflow to help reduce wet spots and mold. Good Luck!