Thank you for the code. The reason why you cannot reproduce the problem because the value of the ControlID property of the AjaxUpdatedControl is not set to the name of the user control. The value of the ControlID property suppose to be "WebUsercontrol1" since that is what you use to name the user control.
Please change the following line in the Default2.aspx from:
| <telerik:AjaxUpdatedControl ControlID="WebUserControl1"/>
Please let me know if you still cannot reproduce it.
The reason why I use the RadAjaxManager on the parent page is only load and show content of user control (using ajax) if the user click on the btnRefreshAjaxManager button (I should have name it btnShowUserControl instead).