Common sense tells us that the cause of suffering is impermanence. We die, nothing lasts. We know this, ergo we suffer.
However, Dogen ascribed the opposite view–ascribed to Senika–that the body and mind/soul are separate, and the latter is permanent, as the root of suffering. The root of suffering.
To make sense of this claim, I think we have to assume that for Dogen, separation–dualism–was the cause of suffering, not impermanence. A belief that we have an eternal essence solidifies dualism. It follows that impermanence has the primary function of waking us up to dependent origination, the dynamic wholeness of everything, waking us up from the dream of suffering.