Board Thread:Questions and Answers/@comment-31718576-20170411140100/@comment-27701762-20170412233830

You've run into the same problem? If so, it might be an error with how the game downloaded. That's the best I can think of, as I did a quick Google search and couldn't turn up anything about anyone else related to what you are experiencing.

Your next solution is one of two things: uninstall the game completely and redownload it, or remove the files for your current run and see if starting a completely fresh run solves the problem. If you go with the former first, you should be able to still hold onto your current save by manually copying the undertale.ini and file# files from the Undertale folder. Save those in a separate folder just to be safe, then when you reinstall the game you can paste back in, letting you start from your last save point.

If that still doesn't work, then you might need to start with a completely fresh save, meaning getting rid of those files. Which means you'd need to complete the Neutral Route again.

If that still doesn't work, then I'm out of ideas.