After I posted the question a student of mine discovered a limitation of Chicken exporter. Only NURBS curves (not circles) with EXACTLY 4 nodes can be exported correctly.
Is there any other solution? Because a curve with only 4 nodes does not help me much.
Hm, it’s strange. I’m not sure about Chiken, but you can try to use YABEE with Blender > 2.5 (I recommend latest 2.62). YABEE can export NURBS with two or more nodes, however limitation about non cyclic NURBS is still in force.