Damn it. I think I got my mistake (at least the test object now is working).
In the Howto from Chris_G:
"1. Attach the sketch to the target face."
I misunderstood that as "a step to do", but as I understand it now, it's more a hint, how to achieve the connection between sketch and surface, if the sketch exists prior to the surface it should be attached to...
(So the first step really to actually DO in freeCAD is point 2 in a way.)
EDIT: Update the Curves workbench. Appologies:
"Did a rework" (-> on the workbench, not only at the tutorial)
"I'll be back"