@Radovan_Prikryl - that’s very odd. If I’m looking at the pictures correctly; it almost looks like you’ve printed the same object twice in slightly different positions - as if you’d duplicated the same object in the same position but with different support heights. Perhaps your STL has some odd surfaces?
I’ve seen supports with contact points like that before when my model had a “submerged” surface that wasn’t corrected properly on import into Preform - although when I looked closely in Preform I could see the support going into the model, whereas your preform screenshot shows it’s all normal.
Do you get the same effect with other objects? Perhaps as David suggests try something standard like the butterfly?