In the meantime, I had continued to work on the opposite side and added a smaller version of a sling there. There the G2 filleting had worked. Then I tried again at the first place - and all of a sudden I could do G2 filleting there, too.
Hmmmm'....
But all in all it makes no difference if the filleting is round or a G*. The transitions are not homogeneous - doing it with my moi-skills.