There is nothing to fix in the plugin. The issue is on your site so lets focus there and find out what it is.
From what you explained above, it looks like you have some remaining PayPal button/form code from your previous usage of PayPal button. You can go to the "Text" tab of the page editor and see those hidden HTML code.
Create a brand new WordPress page and add the add to cart button shortcode and you will see that the shortcode works fine.