The problem for me was not just the diagonal line. In the file, the diagonal line is part of a a shape that is part of at least 2 unclosed shapes that were laid one on another. I could get rid of the diagonal, but then there was an incomplete shape that included what appeared to be a closed shape at the bottom. And the outline exends up to one of the rear view mirrors. When I did a divide, I got yet another diagonal.
I supposed that the curve was originally composed of several lines that had end nodes snapped to ones already existing. It was a major fuss, but I was eventually able to break them apart. I had to isolate 2 different point lines that were on top of each.
Then I could make a discrete shape at the bottom of the body silhouette, and complete the upper shape to 1 closed curve.
OK, there is something I'm not understanding. The shape w. the diagonal appear to be made from 3 shapes, of which at least 1 was not closed. Affinity can't do geometric operations when there are unclosed shapes w/o repeatedly trying to close them.
After a few tries, I'm not seeing how to break them up.
I'm going to go and have some dinner. I'll look at it again after some refreshment.