If a person masturbates on one of the nights of Ramadan, does that affect the validity of the fast of the previous day?

The correct view is that masturbation is forbidden. If a Muslim engages in it during the nights of Ramadan (definition), it does not invalidate his previous or subsequent fasts. However, he must strive against his soul and prevent it from this forbidden act, especially in Ramadan. The Prophet, peace be upon him, commanded young people who are unable to marry to fast, because it curtails desire. And whoever is afflicted by this should repent, regret, and resolve not to return to it.