Instead of setting a person as a child of two parents, I set it as child of only the male parent and afterwards added a partner to that male parent. So the "line" starts from the male parent's box and not from the middle of the two parents' boxes.
After that I added a lot of descendants, so deleting all of that branch of the family tree would be a lot of work.
Is there a way to fix this mistake in another way?
Thanks everyone