Thanks for contacting Telerik support.
IIS6 и IIS7 register http handlers by different ways. We suggest you delete any registrations from your web.config and use the link "Add RadChart HTTP Handler to Web.Config" from Visual Studio Smart Tag-a. For more information take a look at this help topic - http://www.telerik.com/help/aspnet-ajax/registering_the_httphandlers_on_windows_vista_iis_7_integrated_mode.html
Have in mind that RadChart will work in web farm environment under certain conditions. It would be an issue if the page is served and therefore the chart image is generated by one machine and the next request for the image itself is served by another machine. Then this second machine will not be able to find the image in the session. The solution would be to implement State server or SQL server session state mode
or sticky session, ensuring that all the request from one user will be handled by the same server.
the Telerik team
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