Welcome!
I'm not sure how I missed this message the other day. I'm not sure if I would think of resource related performance issue, for this problem. I can think of other explanations for this, and in fact have experienced the same thing myself.
Especially on a complex canvas, there may be many duplicate objects on top of each other, in the case of realism or 3d simulation. Or there may be situations which create a few to even several nodes stacked up on the same point. It's easy not to be sure if you have selected the correct object, or selected the correct nodes -- depending on your selection technique.
If you find this is continuing to happen, would you consider sharing the SVG file with us? Then we can help you to investigate what the problem is.
I'm not familiar with Sheetcam. I looked it up, and it appears to be a program that sends files to cutter/plotters. But that doesn't explain to me why you have a particular need to break the path between 2 nodes, multiple times. I can see the need for that, just as in general node editing, to prepare the path for cutting. But other ways besides breaking the path between 2 nodes.
One thing that you can look at, if you keep having this problem, is the new Object manager (Object menu > Objects). You can use it to help make sure you have the correct object selected. It wouldn't particularly help with node selection, but at least would cut the problem in half, by confirming you have the right object selected.
