Smooth square blurring towards zero could cause KDEnlive to attempt negative square blurring (which is impossible) to complete the smoothness to the next key frame, therefore, it crashes.

Tested with the master branch. I added the effect "square blur" to a clip. Set 2 keyframes and change both to "smooth". Set the "kernel size" of the second keyframe to 0 (zero). No crash. Are these the settings you have?