Hi Matadem, which version of MoI are you using to do the fillet?

Filleting has been worked on a lot in the current v5 beta and your file is filleting ok for me in v5. You can get the v5 beta at https://moi3d.com/beta.htm .

If you don't want to use it for some reason, let me know what radius fillet you want and I can do it for you.

- Michael