Arnoldo,
The scenario like WebUserControls or Master/ContentPages try to set the RadAjaxManager instance on the main/master page and add a proxy to the usercontrol/content page. RadAjaxManagerProxy copies the exact same RadAjaxManager designer configuration so that you can set all the necessary AJAX settings within the WebUserControl/ContentPage entirely through the designer. Please have a look into the following code snippet which works fine at my end.
ASPX:
UserControlPage :
<
asp:Content
ID
=
"Content2"
ContentPlaceHolderID
=
"ContentPlaceHolder1"
runat
=
"Server"
>
<
telerik:RadAjaxManagerProxy
ID
=
"RadAjaxManagerProxy1"
runat
=
"server"
>
<
AjaxSettings
>
<
telerik:AjaxSetting
AjaxControlID
=
"cmbBanco"
>
<
UpdatedControls
>
<
telerik:AjaxUpdatedControl
ControlID
=
"cmbCuenta"
/>
</
UpdatedControls
>
</
telerik:AjaxSetting
>
</
AjaxSettings
>
</
telerik:RadAjaxManagerProxy
>
<
telerik:RadAjaxLoadingPanel
ID
=
"RadAjaxLoadingPanel1"
runat
=
"server"
Skin
=
"Office2010Silver"
>
<
div
class
=
"loading"
>
<
asp:Image
ID
=
"Image1"
runat
=
"server"
ImageUrl
=
"~/Imagenes/loading1.gif"
AlternateText
=
"loading"
>
</
asp:Image
>
</
div
>
</
telerik:RadAjaxLoadingPanel
>
<
table
align
=
"center"
style
=
"width: 98%"
>
<
tr
>
<
td
class
=
"auto-style4"
>
Banco:
</
td
>
<
td
colspan
=
"2"
class
=
"auto-style4"
>
<
telerik:RadComboBox
ID
=
"cmbBanco"
runat
=
"server"
AutoPostBack
=
"true"
DataSourceID
=
"SqlDataSource1"
DataTextField
=
"Countryname"
DataValueField
=
"Countryname"
EmptyMessage
=
"Seleccione un Banco"
OnSelectedIndexChanged
=
"cmbBanco_SelectedIndexChanged"
>
</
telerik:RadComboBox
>
</
td
>
</
tr
>
<
tr
>
<
td
style
=
"vertical-align: top; width: 20%"
align
=
"left"
class
=
"auto-style5"
>
<
asp:Label
ID
=
"lblCuenta"
runat
=
"server"
Text
=
"Cuenta:"
></
asp:Label
>
</
td
>
<
td
style
=
"vertical-align: top; width: 30%"
align
=
"left"
>
<
telerik:RadComboBox
ID
=
"cmbCuenta"
runat
=
"server"
Width
=
"211px"
AutoPostBack
=
"true"
EmptyMessage
=
"Seleccione una Cuenta Bancaria"
>
</
telerik:RadComboBox
>
</
td
>
<
td
style
=
"vertical-align: top; width: 20%"
align
=
"left"
class
=
"auto-style3"
>
<
asp:Label
ID
=
"lblFecha"
runat
=
"server"
Text
=
"Fecha de emisión:"
></
asp:Label
>
</
td
>
<
td
style
=
"vertical-align: top; width: 30%"
align
=
"left"
class
=
"auto-style4"
>
<
telerik:RadDatePicker
ID
=
"dttFechaEmision"
runat
=
"server"
>
</
telerik:RadDatePicker
>
</
td
>
</
tr
>
</
table
>
<
asp:SqlDataSource
ID
=
"SqlDataSource1"
runat
=
"server"
ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
SelectCommand="SELECT [Countryname], [Cityname] FROM [City]"></
asp:SqlDataSource
>
</
asp:Content
>
UserControlPage C#:
Hope this will helps you.
Thanks,
Shinu.