User Sync not working on one site

The WordPress User Sync plugin is working just fine on all but one of my sites, and I can't figure out why not. All sites are hosted on the same server at WP-Engine, and the site URL & key from the master site work just fine on all sites but one. When entering the URL & key from the master site on this troublesome site, I get the "There was a connection problem. Please check the URL and Key of the Master site." message, but the URL & key have been triple-checked and are correct.