extrude along path with assembled lines

[gogosAC]

Hello,

To realize complex cables in 3D,
I have to assemble the parts of lines,
because the lines always lie in a "zero"-layer.

So, after positioning the vertices of the "first" line-part
I'm adding a second one.
After merging, optimizing, welding I thought the "new" object has
to be a (single) line again.

But, if I want to "extrude along path" now,
I'm getting the error:

extrude path: objects must have exactly one surface and no children

With ONE finished line the function works!

I'd like to have this function so often!

Why does the function finish, if I change the window?

Am I doing something wrong?

Greetings Gogo