Hi Tom, I've attached a tuned up version here - I did an "untrim" on your main body to restore it to its original full shape, then deleted the slightly off round corner in the above screenshot and put in a curve blend in there instead and then extruded a new cutting block from that, then did the boolean.

With this attached version you should now have "all systems go" for doing any fillet radius along that spot.

That particular type of thing with things being only just a 5 degrees or so off from smooth tends to make filleting pretty cranky.

- Michael

Thank you for the patience, your informations are really helpful. Impossible to come up with
a solution without it. Will have a look at the file - great !

Everything fine now after reworking the curves.

Dealing with filleting I realized that it is not possible to round with a perfect circle - for example if
you have a distance of 2,00 it is not possible to fillet with radius 1,00 exactly but a bit smaller than
the radius needed for a perfect circle.

Is there a specific reason behind it ?

Hi Tom, the fillet will fail if it would cause a face to be entirely consumed away as would happen in that case. The fillet engine does not currently know how to handle that type of case.

- Michael

Ok good to know, I had speculated that I may have made a modelling mistake.