Is the 'Isha prayer valid if one performed ablution and wiped over his khuffs (leather socks) before 'Isha, then prayed 'Isha on the following day without having removed the khuffs, knowing that he had wiped over them?

It is permissible to wipe over socks if they are thick enough to cover what is obligatory (to be covered). The duration for wiping is one day and one night for a resident, and three days and nights for a traveler. The period of wiping begins from the first nullification of ablution (hadath (definition)) after wearing the socks in a state of purity, according to the majority of scholars. So, if the worshiper nullifies his ablution after wearing the socks, the timing begins from the moment of that nullification. If one prays a prayer by wiping beyond the specified duration, then their prayer is invalid and must be repeated, along with any subsequent prayers.