Sorry but that demo bears no relationship to what I am doing.
However I did have time to create a demo illustrating the bug. (No code-behind needed.)
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<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication4.WebForm1" %>
<%@ Register assembly="Telerik.Web.UI" namespace="Telerik.Web.UI" tagprefix="telerik" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
<Scripts>
<asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.Core.js">
</asp:ScriptReference>
<asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQuery.js">
</asp:ScriptReference>
<asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQueryInclude.js">
</asp:ScriptReference>
</Scripts>
</telerik:RadScriptManager>
<br />
Test form. The content area of the RadEditor becomes read-only if the OnClientSelectedIndexChanged property of a RadComboBox is
<br />
set to a non-existent JavaScript function. Remove that and the RadEditor functions normally.<br />
<br />
This app created with Telerik Web UI version 2012.2.607.40<br />
<br />
<div>
<telerik:RadEditor ID="RadEditor1" Runat="server">
</telerik:RadEditor>
<telerik:RadComboBox ID="RadComboBox1" Runat="server" OnClientSelectedIndexChanged="blahblah">
</telerik:RadComboBox>
<br />
<br />
</div>
</form>
</body>
</html>