I am trying to fillet the outer corner of this object, but it is not working, not even with a smaller radius. I separated it, looked at the individual elements, but can't find a flaw...
Moi V3

this ?

select the necessary lines ... copypaste ... trim to delete parasites segments ... extrude ... filet

it is about this edge which cannot be filleted:

I don't quite follow your described workflow...

problem in your piece ( the small yellow line )

select the segments and join them

then extrude and fillet

file joined

the 2 borders ( in your piece ) are composed with small segments ... you need to use Join to obtain 2 perpendicular lines

and then large fillet can be applied

As corchet said, it is because of that broken edge.

So select any edge, press Ctrl + A to select all and use the Merge command.

Also if you select the whole object and run Merge that will also target all the edges of the object.

- Michael

Thanks all for the tips!

wim