In Exact Surfaces>Edit Contours>Edit Extensions you can click Check Problems to find out if there are any errors with the extension lines.
Vertex errors occur because there cannot be an end of a line (red vertex) in the middle of a curve.
To remedy this error, use your mouse and single click on one side of the red vertex, then drag the mouse to the other side of the vertex. The point will turn yellow or green and it will no longer be an end point. This will turn it into a subdivision point along the curve instead of a vertex.