This is what the first user sees when they try to comment: https://grab.geekpowered.me/NHGy
This is the test post I created: https://geekpowered.me/2016/01/28/test/
In the prior post, I turned off WP Sentry and it allowed people to comment. As an admin I can reply to comments with WP Sentry turned on, however, non-Admin users get the routed to the page showing in the screenshot.
In addition, as an Admin if I try to comment on a post that has no comments, it takes me to the Oops, page not found page, but as a Subscriber, it takes me to a blank page.
WordPress is up to date: 4.4.1 and I am using a custom theme.