You may be trying to reload a save that was in the middle of dialogue, or in an area that I had to tweak the coding for. Ren'py sometimes glitches out when things aren't exactly the way they were when a save was made.
Try loading one of your autosaves (click on the "A" to the left of the page numbers on the load screen), and see if that works. Autosaves are created every time you sleep, so you shouldn't have lost much progress.