Finally i have blended the edges first and then I used sweep (2 rails) in between. Since your geometry is not symmetrical, you cannot mirror the blended edges and you have to to it for every edge again.
What it looks like is that the edges of the blend surfaces do not quite intersect with the existing rounded corner surfaces, so the trim fails… FilletEdge in this case works fine…
There does seem to be something fundamentally buggy with both the BE surface creation as well as the intersections, I can’t even get Intersect to have a correct result - or in some cases any result at all - between the long straight sections, when it should be straightforward… @pascal one for the bugtracker?
Me too, it’s definitely buggy. As BlendEdge actually created the surfaces but didn’t trim them, I ran intersect between one of the corner surfaces and the blend surface. The intersection is incomplete, as the blend surface edge is just far enough away from the surface in spots that it can’t trim.