found it!

that curve was initially a straight line, which was joined to other curves, but I had to change some points, so I separated the joined curve, and the straight line was thereby converted to a freeform curve, and the control lines are at the same place of the straight line itself, which gives a dotted result.
I think I found why the lines would not join, I selected them in front view, and probably had another line behind that selected too. selecting in 3d view solved it.

rgds
wim