No clue exactly when/how this happened. I noticed the game was not generating new notice board tasks. Trying to fix this by saving, closing the program, the loading lead to the game failing to load the save — it would fade out, then fade back in to the main menu. Looking at the save game, the "game_world" property was null for the manual save and all 3 autosaves, so whatever happened had been happening for several game days. Storyline-wise, I had recently killed Fred and lied about why, but I can't say for sure if the problem had started before finishing that.
I was able to "fix" the problem by starting a new game and copying the game_world data from a new game to my manual save (though I wouldn't be surprised if I run into a softlock later). I've copied one of the autosaves that has the null game world if you want to take a look. This happened on 64-bit Windows.
https://drive.google.com/file/d/1qTQGBTeGhQcmjT84EVxtl0i-_veOuKQZ/view?usp=shari...