Hi Jonzker, it looks like one of the previous fillets didn't do a completely proper job and left a little gap in the object.

One clue for this is with version 2.0 the "object type" readout in the upper-right corner of the window says that this object is a "Joined srf" rather than a "Solid" :



The problem seems to be along this edge here:



This object may need a bit of surgery in that area to repair it first before that fillet you want to do will work, I'll take a look to see if I can tune up this object.

- Michael