I find a way (a real boring way…):
-Rotate the part in Onshape to be in the ‘Top Plane’.
-Export the part in IGES format
-Open the IGES in Freecad
-Export as DXF with Freecad
-Open it in Estlcam, it works now
So, the problem seems to come from Onshape. That’s sad, because I only use that now…
At least I can work again, but if somebody have another easier way, you’re welcome.