Using of Fiddler was bad idea: "Report Viewer Configuration Error" html block is a hidden div used from javascript. I issue was with ServerReport.ReportServerCredentials I didn;t check that IsPostBack. Correct code is: if (!this.IsPostBack && !this.IsAsync) { ReportViewer1.ServerReport.ReportServerCredentials = new ReportServerCredentials(); }

I have ASP.NET 4.0 WebApplication with ReportViewer page. I can see ReportViewer with Report Parameters Prompt. But when I click "View Report" I get nothing (if I run my browser in IE7 compat mode I receive JavaScript error). I used Fiddler and found that IIS returns page with following data: <h2> Report Viewer Configuration Error </h2><p>The Report Viewer Web Control HTTP Handler has not been registered in the application&#39;s web.config file. Add &lt;add verb=&quot;*&quot; path=&quot;Reserved.ReportViewerWebControl.axd&quot; type = &quot;Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a&quot; /&gt; to the system.web/httpHandlers section of the web.config file, or add &lt;add name=&quot;ReportViewerWebControlHandler&quot; preCondition=&quot;integratedMode&quot; verb=&quot;*&quot; path=&quot;Reserved.ReportViewerWebControl.axd&quot; type=&quot;Microsoft.Reporting.WebForms.HttpHandler, Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a&quot; /&gt; to the system.webServer/handlers section for Internet Information Services 7 or later.</p> But I have Http Handlers configured as specified here http://msdn.microsoft.com/en-us/library/ms251661.aspx Also I have ReportView Redist installed.