Make sure comments are not closed on your site or on the post in question. Also, make sure you don't have the "only registered users can comment" setting turned on. Look for these settings in the Settings >> Discussion panel of your WordPress dashboard.
Your theme's <body> element likely has a height:100% assigned. Remove this style attribute via css from the body tag and that should fix the issue.
11/2/2018 - 1.1
10/10/2017 - 1.0