When there is a need to open dialogs in browser's window instead of RadWindow, the UseClassicDialogs property is being used. With the release of Q2 2015 the functionality stopped working and throws JS error when a dialog is opened. 

To workaround this behavior you can apply the JS patch as shown below:  

    <telerik:RadEditor runat="server" ID="RadEditor1"></telerik:RadEditor>
    
    <script type="text/javascript">
        Telerik.Web.UI.RadDialogOpener.prototype._applyParameters = function (dialogName, container) {
            //If no parameters provided, return
            var parameters = this._getDialogParameters(dialogName);
            if (!parameters) return;
    
            var parametersUrlPart = "&dp=" + encodeURIComponent(parameters);
            var baseUrl = this._getBaseDialogUrl(dialogName);
            var totalUrlLength = baseUrl.length + parametersUrlPart.length;
    
            var parametersPassedFromClient = this._dialogParametersProviderTypeName == "";
            var parametersPassedThroughUrl = parametersPassedFromClient && totalUrlLength <= this.get_dialogUrlLengthLimit();
    
            if (parametersPassedThroughUrl) {
                //NEW: If the dialog features the same URL, do not reload it
                var curUrl = container.get_navigateUrl();
                var url = baseUrl + parametersUrlPart;
    
                if (curUrl != url) {
                    if (this.get_useClassicDialogs() || container.isCreated()) {
                        container.setUrl(url);
                    }
                    else {
                        container.set_navigateUrl(url);
                    }
                }
                else //Reinitialize
                {
                    var contentFrame = container.get_contentFrame();
                    if (contentFrame && contentFrame.contentWindow && contentFrame.contentWindow.$find) {
                        //TODO: Probably implement a global function initializeDialog that will be called automatically - easy for custom dialogs
                        var initDialog = contentFrame.contentWindow.initDialog;
                        if (initDialog) {
                            contentFrame.contentWindow.setTimeout(function () { initDialog(); }, 1);
                        }
                    }
                }
            }
            else {
                container.setUrl(baseUrl);
                container.DialogParameters = parameters;
            }
        };
    
    </script>