Answer:
During the dxf import, only objects lying in the XY plane are imported. Therefore, it is necessary to draw the cross-section in the XY-plane in the CAD application. Image 01 shows a cross-section lying in the XZ-plane as well as the import result of this cross-section in SHAPE‑THIN.
The cross-section lying in the XY-plane is imported into SHAPE‑THIN as desired (Image 02).