This error message is displayed when Silverlight is not installed on the System.
We ran into a scenario with a customer where they could not install Silverlight on the user’s machines and also had to avoid the mixed content error.
After further research we found that SharePoint provides a configuration to control this. The Web Application has a configurable property “AllowSilverlightPrompt” that will control if the download Silverlight prompt can be loaded or not. By default, this is set to true.
We ran the below PowerShell to change this to false and this fixed the mixed content error message.