You are the first to report this problem and we are not familiar with it. I found the following forum which could be helpful: Validation of viewstate MAC failed
the offered solution is to set enableViewStateMac
in the web.config:
<pages enableViewStateMac="false" />
If this does not solve the issue, please isolate the problem in a sample working project and send it for examination through a support ticket.
You can also try to replace RadEditor for ASP.NET AJAX
with its Telerik Editor for MVC
the Telerik team
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