Hi Wim - the geometry engine that MoI uses tends to have difficulty with fillets that shrink down to a single point which is what you have here.

But it seems to work better in this particular case if you only do one fillet at a time instead of all 4 at once, so I'd recommend trying that.

If you're still stuck please post the 3DM model file with your particular cylinders in it and let me know what radius you want to use for the fillet and I will try to help you finish it.

- Michael