XMLHttpRequest.open() causing mixed content warning

Question

This is on a sony TV browser which is using Opera 12. I stopped it in a debugger and as soon as the xmlhttp.send() gets called, the warning shows up on the TV: "this page contains both secure and non-secure items. Would you like to continue?"

I can't, for the life of me, figure out why that would cause that error, before it even gets a response. This is all done on https.

request in this instance is: https://COMPANYURL/vendors/vendor/stores?zipcode=80002&limit=10

MicroServiceDataSource.prototype.fetchReply = function(request, successCallback, errorCallback) {
    var that = this;
    var xmlhttp;
    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    }
    else {
        // code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.open("GET", request);
    xmlhttp.setRequestHeader("Authorization", "Basic " + btoa(SceneManager().appConfigModel.microserviceUsername + ":" + SceneManager().appConfigModel.microservicePassword));
    xmlhttp.send();
    //error happens here ^^^
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4) {
            if (xmlhttp.status == 200) {
                var replyObj = JSON.parse(xmlhttp.responseText);
                that.handleSuccess(replyObj, successCallback);
            }
            else {
                that.handleReadyError(xmlhttp, errorCallback);
            }
        }
    };
};