Article relates to
RadEditor for ASP.NET AJAX
The problem is caused by the following line in project's web.config file:
This setting is incompatible with ASP.NET AJAX controls (like RadEditor) and it should be removed. This setting could be added when upgrading an old project with Visual Studio.
You can find more information about the <xhtmlConformance mode="Legacy"/> setting in the following blog post: Don't use <xhtmlConformance mode="Legacy"/> with ASP.NET AJAX.