Hi Gunter, a few notes -

The reason why filleting doesn't work on your ring is that these areas are not smooth, they are slightly creased:





Areas that are not smooth but fairly close are difficult for the filleter to handle. That's because fillet surfaces only naturally touch each other between surfaces that are tangent to each other. When there is a crease the fillets do not end in the spot and will need to be extended and intersected with each other which is a difficult process between surfaces that meet at a shallow angle.

So to make that able to be filleted, you would want to be more careful about how the profile curve is constructed so that it is all smooth before constructing from it.

For Phiro's result I think that was done with using Sweep. To reproduce it, select these 2 curves and run Construct > Sweep:



Then select these 2 as the rails and push "Done":



That should generate this result:



- Michael