I did not add or remove any mods initially (though I did both while I was troubleshooting the problem, and only by removing EVERY mod did the error go away. It also returned when I enabled a single mod, regardless of which mod I tried).
Also, I don;t know why I didn't notice it before, but when the error is going to happen, the scene thumbnails are blank. Going into Prepare Mode for the related campaign and then loading the save loads the thumbnails correctly, and does not get stuck loading. Notably, this is the case even for scenes that are not present in the Prepare Campaign mode, as they were added during play to the save.
