Thank you very much for your help. This indeed worked, the export displayed correctly in Blender and the Cura prepare tab. However after slicing, Cura wants to print the thread area as a complete block. This might be a Cura issue but could also be a hint that the exported file still is not quite right. Find the Cura topic about this issue here:
https://community.ultimaker.com/topic/2 ... -as-block/
Also, how do you get this sliced view as visible in your image?