Server Error in '/' Application. The control with ID 'RadTextBox1' requires a ScriptManager on the page. The ScriptManager must appear before any controls that need it.

Solution: This error occurs when you have added a control from RadControls for ASP.NET AJAX to a page which does not contain a ScriptManager control. To resolve that error please add a ScriptManager control to your page:

[ASPX] Add the following line in the Master Page using Sharepoint Designer 2007.

<asp:ScriptManager ID="ScriptManager1" runat="server" />