Sorry for such a late reply, but I took a break from the game after trying out the boss fight for the first time and had played around 8 hours in total already in that one save file. However, when I tried to open the save file, that was the message I got. I had no trouble before with the opening and saving and I didn't necessarily try out anything in-game that would have messed with any files or gameplay, so it was not an error from mid-game. I left the game at just a random spot too, so it was not in the middle of anything.