As I understand it
An operator error would be somebody pressed the wrong key or miscalculated the delta-v or punched in the wrong burn time on the engine.
A fundamental design problem is a problem that simply cannot be fixed. Somewhere along the line that your fuel of choice produce a chamber temperature which is borderline of what you can handle and sometimes it doesn't work. Meaning you have to either add active cooling and thus adding too much weight or change fuel, but that would mean loosing specific impulse so thus reducing the weight limit. Which means you have to basically start over.
They are not claiming this is an act of god, it is a design problem, but can probably be solved with a pound's worth of insulation.