Turns out it was the router. At some point, for whatever reason, I had paused the raspberry pi's internet connection through the Google Home app. I apologize for wasting your time on this wild goose chase, and thank you for your extensive help. I kind of feel like a total idiot, but I'm leaving this here for someone else who might have done this and forgotten, and gotten crazy cryptic error messages (at least to a relatively untrained eye).