Hi Wim, looks like a bug in the curve boolean processing, sometimes it can get confused about which loops should be kept.

In the meantime to finish your object and avoid that particular bug you can use the Edit > Trim command to cut your curves up rather than the boolean.

When you use Trim zoom in closely to the ends of the cutting curves they are sticking out just a very small amount beyond the main shape and so there will be a couple of pretty small segments to trim from there as well.

- Michael