The knife tool on X8 (64 bit) works inconsistently. After I start the program I can cut lines. Then the knife tool stops working. It won't cut. I have switched back to X7. Very disappointing software.

I always thought the knife tool was used to slice a polygon into two polygons.  What I mean is that I thought you had to draw a line across two different line segments to split the polygon into two polygons.  Or, with snap on, draw a slice line from one node across to another node, thus, splitting the polygon into two.   I've never tried slicing just one segment.   For that, I would simply add a node to a segment, and then split or separate that node, then you can  move those two open nodes around.