WP Smush Pro - Request failed Site URL not accessible

When I run WP Smush Pro on my website, I receive the following error: "Request failed Site URL not accessible." I commented on this issue on a different thread with the same topic (https://premium.wpmudev.org/forums/topic/request-failed-site-url-not-accessible#post-806339) and was asked to start a new thread on the subject. I'm running WP-Spamshield, W3TC caching, and the Wordpress Photon CDN. Are any/all of these interferring with WP Smush Pro? Thanks.