Opt-in confirmation page rendered as plain text, not HTML

Whenever a user clicks a link from an email, the page they land on is rendered as plain text instead of HTML. Because of this, the 10 second redirect to the home page isn’t executing and they are left on a unrendered page.

Here’s an example URL that shows the issue:
https://anointedcourage.com/?es=optin&db=4&[email protected]&guid=tfecyx-ohgqmz-otazjb-stdnph-sirvag&cache=uqsnwt-qzojfa-qopftc-pgerxu-fzdyum

Running WordPress 4.6.5 with ES 3.2.10 on HTTPD 2.2.15 in CentOS 6.6.

Please let me know if you have any questions or need any additional information.