This error is due to missing configuration option for the Bing layer. The settingsUrl property of layer options could be used to fine tune the URL and modify it to request data from SSL enabled URL and resolve the mixed content issue.

To resolve the issue, you can configure this property on the OnInitialize event and the options of the Map, exposed by the arguments:

<telerik:RadMap runat="server" ID="RadMap1">
    <LayersCollection>
        <telerik:MapLayer Type="Bing" Key="YOUR BING KEY"></telerik:MapLayer>
    </LayersCollection>
    <ClientEvents OnInitialize="OnInitialize" />
</telerik:RadMap>
 
<script type="text/javascript">
    function OnInitialize(sender, args) {
        var options = args.get_options();
        options.layers[0]["settingsUrl"] = "https://dev.virtualearth.net/REST/v1/Imagery/Metadata/#= imagerySet #?output=json&jsonp=bingTileParams&include=ImageryProviders&s=1&key=#= key #";
        args.set_options(options);
    }
</script>