The vtkContourTriangulator has a bug where it sometimes fails with collinear points (like points 3, and point 6). It will succeed if you remove such points, but that is probably not the general solution you’re looking for. VTK’s Delaunay triangulation also has problems with such points.
I’m aware of the bug and know approximately where it is in the code (a particularly tricky part of the code), but haven’t found time yet to dig in and fix it. Examples like yours are very useful to the debugging process.