You can try also the following troubleshooting steps:
- See how to integrate Telerik UI controls in a webfarm scenario in this blog post - http://www.telerik.com/blogs/integrate-radcontrols-for-asp.net-ajax-in-a-webfarm-or-webgarden
- Use a sticky session to ensure the client request is handled by the same machine or configure a shared session with SQL server or session state server. See this forum for details - http://www.telerik.com/forums/chartimage-in-webfarm
- Make a further investigation regarding the session sharing between webfarms on the Internet - https://www.google.bg/search?q=share+session+between+webfarm&gws_rd=cr&ei=BuuxVuDJIoezsQGH7JfoAw
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