Hi PaQ - also one other workaround method is to use surface/surface filleting instead of edge-based filleting, the surface/surface fillet will be constructed in this type of situation.

To use surface/surface filleting, the objects involved must be individual single surfaces instead of joined-together objects. To get that you could select the 2 surfaces (sphere and the main body piece), then push Edit/Separate once to break that 2-surface chunk off from the main object, then push it again to break it out into 2 completely individual surfaces. Then you must deselect those objects and select each surface with a click - surface/surface filleting needs a click selection point on it to know which fillet to build in cases like 2 planes crossing over each other like an X.

That will build the fillet, then you can join those pieces back together.

Surface/surface filleting can succeed in places where the edge-based filleting will fail, the edge based calculations are more complex (but also more convenient because they know how to build corner pieces where different fillets meet).

- Michael