SSL HTTPS troubleshooting
Both the theme and plugin should work just fine in SSL / HTTPS. It could be that something has not been fully setup yet for SSL on your site. Have a look at this article that displays the steps for setting up SSL in WordPress: http://www.wpbeginner.com/wp-tutorials/how-to-add-ssl-and-https-in-wordpress/
Let me know if the issue persists after ensuring all the steps have been followed.