Set Featured Image Not Working

Set Featured Image is not working on posts. If I disable the iThemes Security plugin, the ‘Set Featured Image’ feature works. I’ve tried to disable almost every feature of the plugin I could find, but no luck. I get a weird “blank image” as the first image selected when I try to set the image; although, selecting any other image doesn’t work anyways. Any thoughts?

https://www.ads-software.com/plugins/better-wp-security/