Hello Phiro,
Thanks!
“A rebuild” or “reconstructcurve” don't seem to do anything for me. But maybe I'm not selecting the right lines. Or do I need to install a script first? Because otherwise, I'd have to click on every single line segment one by one.

Hi Paf,
Thank you so much for your great dedication and for helping me with this. I'll try Phiro's approach first, and if I get stuck, I'll try yours.

ok - ready. Thank you very much!