The message "Cannot find differing model" is not an error displayed by RuffRuff Order Limits. It appears when you are viewing the theme in preview mode and is not shown on the live online store.
Check the behavior of the product page and cart page with the theme published rather than in preview. You can ignore what is displayed in preview mode.
If the same message appears on your published store as well, contact us through the inquiry form. Please let us know the URL of the page where it appeared and the name of the theme you are using.