That's why @JanReilink specifically mentioned "the hosting property www_root", there's no ambiguity there.
It happens with every Windows path, even when first requesting the current value of www_root, and then trying to set that very same value. (which obviously shouldn't happen — the current...
That response makes no sense.
www_root is the identifier of the property in question, it's not something you can change at will. It's the value of this property that is rejected by Plesk, not its name.