So this is really testing my patience at the moment.

I have drawn a triangle with a width exactly that of the helix pitch, yet am getting this error code. If I make the width a fraction smaller, it rebuilds fine.

Can someone tell me why on earth it is doing this please!

The problem is it becomes a self intersecting zero thickness geometry as it completes a helix (the top left and right points of your triangle effectively merge and it attempts to cut the vertex twice (once from the left then on the next rotation from the right) but the second time the vertex isn't there. The only real option I have found when cutting threads that need to be go to a point is to undersize the cutting profile fractionally on one side so you don't go to a perfect edge but a minutely thin face, then do a second helical swept cut to remove the tiny remaining part of the cut (You could just cut the triangle in half thinking about it now, do the left half then the right half) it's a bit of a pain.