The new release 3.6.16 includes an old version of the CKEditor plugin, 2.6.9, which does not handle the warning about not being secure.
If you have already upgraded the plugin to remove the warning about being insecure as explained at CKEditor shows warning message about being insecure - #8 then you will need to repeated the upgrade. The Manage Plugins page should indicate that the latest version is 2.8.1.