Hi niko, also this kind of thing here tends to be kind of bad for sweep in both MoI and Rhino:




Your profile curve there stretches out quite a bit and is tangent to where it touches the rail, which is very unlike the other profile curve on the other side.

This creates a surface with control points really kind of compressed in that corner area:



This point here is actually the corner point of the NURBS surface point grid:



It's very easy for such things to have bad surface qualities in that area, with the surface actually being bunched or folded back over on top of itself right in those corner areas.

You may want to do something like put in a surface blend here and then trim it to make it rounded at the end rather than trying to sweep in this way.

- Michael