I haven't checked recently when exactly YouTube is updating encryption (because everything is automated now), but in the past, they would generally do this 2-3 times during the week, Monday through Friday. Never on weekends. Usually on Monday/Tuesday, Thursday/Friday.
So it's possible YouTube had "just" changed encryption prior to your download error, and then the auto-update feature immediately fixed the problem. So you were at the right place, at the right time...;)