For whatever reason, your saves were generated in a version of the game running in RenPy 8 (or *maybe* 7.5), which means they won't load in the game when it is being run with older versions of the engine prior to 7.5 (e.g. 7.3.5).
To get around this, download the RenPy 8.x SDK and run the RenPy app. Figure out where the included demo games are stored on Windows (no idea) and drop the entire game folder into that same directory. Then click the button to refresh the list, and you should see this game appear. Click the name of the game and click the button to run it. You'll have to run it that way from now on, but it should fix the problem.