Sweep has unexpected result

 From:  NaN
10575.1 
Good evening!

Trying to get a thicker version of this polyline.


For that I try to sweep a circle with a fitting radius around it. The result is not what I expected, the sweeped body doesn't follow the polyline but goes all over the place.



Is there a way to just use the polyline as rail for a circle in order to get a thicker version of that polyline?