Hi Jürgen,
just to inform you - I had worked with Illustrator DXF files before and just tried to import a text set in »Helvetica Neue - Medium« without issues. The letter forms look ok and I could extend them without issues. Today I used the latest Illustrator (v25.0) and the actual Shapr3D.
Did you try to open the DXF file in another CAD app just to check, if the DXF is ok?
By the way, your »Lego« usable font looks great and I like the idea. I wish you much success with your »Cy« font.
Urs