Hi there! Sorry for the long absence.
I think the problem might be the "rendering order" in the "Edit Controls".
The error message is telling you that the point that you are selecting is not connected to an edge. This is because what is selected is the center of the endpole, which is at the same location as the endpoint of the bspline.
When there are co-located points, the point that is selected, generally depends on which object was created first, and thus is rendered first, unless one of the specific rules of the "rendering order" applies.
You may remember that the original B-Spline creation introduced the circles of the poles AFTER the B-Spline. Then you always had to use the work-around of using the Element's widget. Then the creation was rewritten some six months ago to introduce the circles of the poles BEFORE the B-Spline. This allowed that, with the "normal" rendering order, the endpoints are selected and it is way more usable. However, if one changes the order to "construction" geometry first, then you will select the center of the construction circle of the pole.