I appended an example file with a selected bone that has an IK constraint. Since I'm using iTaSC i can deactivate "rotation" and "position" in the constraint panel. The problem is that Blender will always crash if i deselect both options (both on = fine, one on = fine, both of = crash). I can't reproduce this error with a more simple scenario. It seams to happen when two IK-Constraints deform the same chain (tree ik).