I say, and with God's success: It is invalidated; because the removal or taking off of either one or both of the socks nullifies the wiping (mas'h); due to the departure from the place of wiping, as the removal causes the event (hadath) to spread to the foot, and the sock was what prevented its spread. Therefore, if it is removed, it is invalidated, even if only one sock is taken off; because if one of the two socks is removed, he must wash one of the feet, and thus he must wash the other, as there can be no combination of washing and wiping. See: Sharh al-Wiqayah p.116, Radd al-Muhtar 1: 183, and God knows best.