Check if they are root bound, if so, plant in a bigger pot. Make sure they get enough water, but not too much. Get a water meter $20 at your hardware store or garden

Soil water probes are very innaccurate. Learn how to assess soil moisture by appearance, weight of the pot and, if necessary, using a ’tell’ ie something like a chopstick pushed into the soil. (Judging by the roof in the background I don’t think Aleksandra is in a country which uses dollars). I’m guessing those trees are just coming out of a fairly chilly European winter. Could they have suffered cold damage?

Thanks for the advices.
You right, I live in Munich, but the winter wasn’t too cold, I warmed pots up with a fleece though. May be it was too warm for them and the water didn’t evaporate properly and the roots rotted. Anyway, I will repot the trees and check the roots.