Hi Heiner, so after a quick look I noticed some of the edges in your C and G looked a little peculiar, and the reason why is that there are many "naked edges" in it, meaning that many places where it looks like the side walls and the top cap are touching each other, they aren't actually joined at a lot of those spots. They're only joined in some areas like at the short leg of the C and not along the whole long curved edges of the C. Similarly with the G there are some unjoined edges.

Was your object possibly initially constructed at some very small size or something? I'm not really sure how you ended up with those unjoined edges, but that seems to be the problem that is getting in the way of your filleting.

It seems that the side wall pieces and the top cap pieces are not quite accurate enough to one another to properly join at the default join allowance. One quick fix is to scale the objects down by 1/10 in size, then separate and join them, then scale back up again. That's what I did with the attached version which should now fillet better for you.

- Michael