wynnhall

The “Filter Suspicious Query Strings in the URL” option is what was blocking the plug-in. Enabling iThemes with that option disabled in the Security section fixed the issue.

There is even a warning that the option can block certain plug-ins.

Thank you Benjamin!

Sure enough, iThemes seems to be the culprit. Disabling iThemes seems to have resolved the issue as a more permanent work around.