The key is mainly for the admin app but the live site cache items are coming from the live site app. I would recommend using valid SSL certificate for security reasons. If it is some testing or development environment, you can try workaround by adding the following line to either the Global.ascx.cs file in your admin project or in a custom module initialization/loader:
ServicePointManager.ServerCertificateValidationCallback = (request, cert, chain, errors) => true;