Known issue I don't know how to fix. It's a Ren'py thing. My apologies for the inconvenience caused.
Viewing post in Sensei Overnight 0.5.0 Part Two Release! comments
The same thing has happened with other games, often when the devs have switched to a newer version of Ren'py. They sometimes knew about it and said that the old saves wouldn't work. Once, one game (My New Family by Killer7) required rolling back a few scenes to reset it, he warned us about it ahead of time. If this isn't happening with everyone, it's a little puzzling. If I have to start over again, well, I'll just get to enjoy the game again from the beginning. Not the first time it's happened. Thanks for your reply and your apology, even though I don't think that it's your fault. Best wishes and thanks for your work on a very entertaining game.