Surely because it's too large take a smaller one! ;)

As we can see on the top image there was calculated ...but overlap...existing solid...
so a smaller one will be fine!

Sometime when faces created overpass the solid you can make a Boolean Diff !

In any case in this sort of object make the fillets one by one it's more prudent!

PS post the object is also a good thing! ;)

Hey Pilou, thanks I tried a minimum of 0.5mm which WAS too large! 0.1 worked!

And sorry, I didn't post the file, I normally do :)

Much appreciated, and on a Sunday too!
Gunter

If the fillet fails like that...you can use the surface for a Boolean Diff! ;)

Great, thank you Pilou, appreciate all you help!
GUnter