Please note that in order to have the RadAjaxLoadingPanel appears corectly you should set the Skin property for the control.
Also could you please elaborate a bit more on your application and the exact approach you are using for updating the Page A content after closing the popup window? Are you redirecting back to the main page or you are calling ajaxRequest for this purpose?
If you are redirecting the page back please note that in Ajax scenarios the usage of Response.Redirect() method is not supported but you should use the RadAjaxManager.Redirect() method which is described here.
If the problem still remains please provide us some more information on your project or send us the full markup of the page as well as the related code behind. Thus we will be able to further research on the problem and do our best to provide proper solution.
All the best,
the Telerik team
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