The browser warns of mixed content when using the measurement tool. Depending on the browser being used, the warning may be at the end of the address bar as a shield icon (Chrome) or as a popup window at the bottom of the page (IE).
The viewer is configured to use a geometry service using HTTP instead of HTTPS. Using Fiddler ( https://support.geocortex.com/customer/apex/essentialsGSCkba?sub-nav=kba&main-nav=essentials&#!/feedtype=SINGLE_ARTICLE_DETAIL&criteria=BESTANSWERS&id=kA360000000ChxJCAS ) would be the quickest way to verify this issue. Depending on the version of Essentials and viewer(s) you are using, you may have to change this setting in a couple of locations:
1) for Essentials, a geometry service is set via the GIS Services page of the main site configuration.
2) For the HTML5 viewer, the Application page in the viewer configuration sets the geometry service. (pre version 2.5)
3) For the Silverlight viewer, current (2.x) versions takes on the site's geometry service.
Older versions may have configuration on the Map Module page. If the viewer was created with version 1.X there may be settings in the viewer.xml that need to be updated.