Hi Jim, so here's the kind of coplanar fragments that I'm talking about (the ones selected here):



Those pieces are all on the same plane as each other. If those are combined together to be just one large planar surface like this:



Then the filleting in this case should work a lot better. It just kind of happens to be an area that the fillet engine that MoI uses is kind of sensitive to.

The way those areas can be combined into one large plane is by deleting them to leave a large hole there, then select the object and run the Construct > Planar command which will build a face on any planar holes and join it in to seal up the hole.

I'll see if I can make a tuned up version of this object for you.

- Michael