Before I make an assumption that the S-400 is junk I too would want to look a little deeper into the subject. See if Ivan turned it off, or the Syrians were busy at a Goat Orgy first.
Another possibility is that someone found a software vulnerability and inserted a virus into the C-cubed-I system causing them to not fire anything. (The Israelis did this about eight or nine years ago.) The Russians certainly would not say anything as it would confirm a system vulnerability to countermeasures. We would not say anything either.
So no missiles are fired and no one admits anything. Sounds like a system failure but could be much more specific than “lousy system”.