Hi telnoi, fillet of a large radius going around a tight bend can generally be problematic.
Normally the way that I would recommend doing this is to start with the full blocky shape like this:
Then do the large radius fillet first:
Then follow with the smaller radius fillets afterwards, that order of large first small later generally helps avoid the kind of bunching that happens if you try to make a large radius go around a tight bend:
Unfortunately MoI's filleter currently runs into problems in this kind of "vanishing fillet" type situation where the fillet surface narrows down and ends in a single point like this (this is when the fillet surface does this, which is different from a corner piece between fillets doing it).
So this will require some kind of low level surgery to fix up.
What I did was to delete those messed up corners, and also the long fillet that was next to them.
Then I selected these 2 open edges:
And ran Construct / Blend to create a blend surface between them:
Then I selected the edges around the top open part and ran construct/Network to fill in that area.
There was another issue when repeating this for the 2 other sides, that the fillet had ended up trimming the edges in that area creating a small sized extra edge in there, so I had to select that edge and its neighbor and do Join before doing Network on those ones to get the full length in the Network.
The final result is attached as lastissue_2.zip, I hope it may be sufficient for you.
I am nearly ready to apply a geometry library update to the v2 beta, it seems to incorporate quite a few fixes for filleting so hopefully this should improve shortly. I'm not sure yet if it covers this kind of "disappearing" fillet surface to a point yet or not though.
- Michael