Hi Jonzker - the regular filleter really does not like this shape very much, it seems to have a lot of problems with the area that is sloped out the most.

I was able to sort of deconstruct and re-construct the object by using surface/surface filleting and some manual surface trimming in that area that the regular edge-based filleter did not handle properly, I have attached the result here as question2.3dm .

This version should now be able to fillet that planar side, you can select the face there:



And then a fillet smaller than radius 1 (like 0.8 or something for example) should work on this version:




Looks like definitely something that needs to be tuned up in the filleter but I hope this tuned up version here will help you complete your current task at hand.

- Michael