Found - finally it was a container mismatching...

Viewport.master:
        function showTab(url, title, id){
            var myTab = #{MainTabPanel}.getComponent(id);
            if (!myTab){
                myTab = #{MainTabPanel}.add({
                    id : id,
                    title : title,
                    closable : true,
                    layout: "fit",
                    autoLoad : {
                        showMask : true,
                        url : url,
                        params : { <U>myContainerId</U>: id },
                        mode : "merge",
                        scripts: true,
                        maskMsg : "Loading..."
                    }
                });
            }
            #{MainTabPanel}.setActiveTab(myTab);
        }
controller:
        public ActionResult Index(string <U>myContainerId</U>)
        {
            QuotationViewData myQuotationViewData = ViewDataFactory.CreateBaseViewData<QuotationViewData>("Quotation List");
            myQuotationViewData.QuotationList = service.GetAll();
            
            Ext.Net.MVC.PartialViewResult myPartialViewResult = new Ext.Net.MVC.PartialViewResult();
            myPartialViewResult.ViewData.Model = myQuotationViewData;
            myPartialViewResult.ContainerId = myContainerId;
            myPartialViewResult.RenderMode = RenderMode.AddTo;


            return myPartialViewResult;
        }
The parameters must have the same names...

Thank you - it can be marked as solved now ;-)