Hi GonzoRus, your outer curve there has difficulty with fillet because in one spot it contains a tiny little line, here:





Curve filleting on this shape will only work with a radius smaller than will fit on that little line. I found the little line by using Edit > Separate on your curve, then going around and clicking to select each large visible piece, then delete those or invert the selection.

If you want to fillet at a large radius you will need to remove little tiny pieces like this from your curve. I've attached an updated version of your file where I removed that tiny segment and moved the endpoints of the neighboring ones together to reclose it, and this one will now fillet up to a radius of 4.8 or so, that's when it runs into the next smallest piece. If you wanted to fillet at a radius larger than that you might try doing it in stages and selecting just some of the sharp corners to fillet at any one time until you find the areas where you would have to rework the small segments in order for a larger radius to fit.

- Michael