Thank you so much for your help.
Extracting the geometry is not the problem.
I tried the way with Element.Geometry as well.
And I played a little bit with the list. Now generally that works.
But still the solid doesn't want be created.
At the moment it always says that the PolyCurves are not closed:
But to be honest I have no clue how to close them?
The only node, which seems suitable is the PolyCurve.CloseWithLine node.
If I use this, and check, if everything is closed by using Curve.IsClosed node, then the result ist, that indeed everything is closed. But then I receive the error messagt, that the wire intersects itself:
I also tried to create a surface out of the closed polyline and then I wanted to create a solid between these surfaces.
But this also doesn't work.
I keep on trying out anything and will keep you informed, if interested...
Thank you all!!!