Beste Wim,
Maybe you use a special stl exporter in SU, but when not, I tried it as well in SU with export to obj: maybe you haven't checked some boxes with the option to export the .obj file (as it wasn't at default with me neither).
After I had checked all of the checkboxes when exporting the obj (at least the triangulation is necessary I suppose), I was able to load it into the free repair program Netfabb. Let it repair and export it as stl, import it in Kisslicer, rotate and scale as you wish.
When you are new at Netfabb: it's a real nice free repair and check program. After downloading the free version: import the obj or stl in it, select it and push the 'red cross' button at the menu bar. Do an automatic repair, default repair, apply repair, remove the old part (or not), export the part (by ricght clicking on it) and export as stl (maybe another repairs are given when exporting it).
After that it's most of the time good enough to slice properly in Kisslicer.
Another free modeler program to consider, which is intuitive to use to shape stl's is the free Meshmixer. But for more precise modelling is SU working as well.
I started with SU as well two years ago, but's not very good to export manifold stl's, so I switched to a paid modeler, ArchiCad, I use for work as well.
There some threads at the old Makerbot forum as well regarding the choose of a modeler,
Bart