Hi wim, it seems to be a bug in the Planar command - what happens is after the sweep the resulting open edges are just a teeny tiny amount out of tangent to one another. There is a mechanism in the curve gathering method that PlanarSrf uses that looks for near tangent curves and if they are really close to being tangent to one another (like within 0.5 degrees), it adjusts the shape of the end tangents so that they are exactly tangent.

The problem is in your case here that adjustment happens on the long straight piece and it changes the shape enough that it's about 0.03 units away from it's original shape and then out of join tolerance with the original edges.

So anyway that's the source of this particular trouble - I'll need to look into whether this end tangent modification should just be removed or maybe if it should test how much the tangent moves the curve away from its original shape and not do it if it's too far...

The workarounds above will get the job done for this case for the moment since they will run through a path that does not do the tangent adjustment.

- Michael