Live Preview not loading - https / http conflict
I had the same issue! 

In my case, it was because I had defined http://example.com as my siteUrl for the production environment, in my general.php config file. Simply changing this to https://example.com fixed it! 

Sample code: (Change 'production' to match the env you are targeting of course)

'production' =&gt; [
    // Base site URL (if multisite)
    'siteUrl' =&gt; [
        'mysite!' =&gt; 'https://example.ca',
    ],
    //OR Base site URL (if single site)
    'siteUrl' =&gt; 'https://example.ca'
],


More on the general.php config file: https://docs.craftcms.com/v3/config/config-settings.html