Hi Wim - switch to the top view and hit the Reset button on the bottom toolbar to zoom to fit, and you should see the curves are on different levels in Y - although they are both planar curves they are on different parallel planes from one another. So their actual 3D endpoints don't touch, that's why they can't be joined.

You will need to go to the top view and move one of them to the same level as the other one, then you can join them.

- Michael