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If you, like me, saved right when the last build ended, you need to load a save from an earlier scene. 

This is unfortunately quite common in Renpy-based games as they get updated.

As stated below unfortunately when we update parts of past chapters it can cause the above error you have encountered to happen. I've seen it in every VN I've read myself and have yet to see any fix for it.
However you may use the Chapter select button on the main screen to jump in where you'd like to start reading from.