The problem appears if you have set the enableSessionState attribute of the Page directive to false. Due to the fact that RadChart stores the image it creates in the session variable, an exception occurs.
Fortunately, there is a solution for that situation. You should set the UseSession property of the RadChart to false, create a folder ~/Temp and set write permissions to it. More detailed information you can find here: http://www.telerik.com/help/aspnet/chart/?chrtDisableSessionUsage.html
You can use ~\Common\Templates\Polls\ResultsTemplate.ascx or ~\Common\Templates\Polls\ResultsTemplateNoLegend.ascx to set the UseSession property of the RadChart. The first template is used if the PollBox -> ResultsBox UseLegend is set to true (poll answers results are displayed in one image), and the second one when that property is false (poll answers results are displayed in separate images).
Hope this helps. Let me know if you need any further assistance.