Incorrect Password at Checkout

I am using the plugin on this site: https://theloveofpeople.com.

When a person tries to log in during checkout, it works perfectly if they have the correct username/password. However, if they happen to put in the wrong username/password, they are directed to a “page cannot be found” error page instead of receiving the “Incorrect username/password” error message. What could be causing this and how do I fix it? The cart and checkout pages are separate.