The old ASP.NET WebForms ReportViewer depends on the ASP.NET Session and ASP.NET Cache in order to preserve reports' resources between requests.
Please check if the application's session state mode is configured based on the hosting, and if reports cover the requirements for the specific session state mode. Also check if the browser accepts cookies. More details are listed in the Report is unavailable or session has expired error message
Note that IE8 is not supported as it is not HTML5 compliant browser. The Reporting engine uses fallback mechanisms to render reports for preview in non HTML5 compliant browsers, which does not guarantee the browser can execute the viewer control's scripts. For more details, please check Design Considerations for HTML Rendering
I hope this helps.
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