I was able to use Cut Plane on your shape without crashing...
TestCounterCut.mdl
There is an easier way to get a 45° angle on an extruded shape such as yours...
clip5034RotateToAngle.mp4
I can't seem to reproduce that crash. Could you give the steps you are doing to get it to crash?
In the video you can see what I tried out.
Best regards
*Fuchur*
plugin_cutplane_test.mp4