Could you inspect the Ajax request using some of the browser's dev tools and verify if any errors appear and if the ajax request properly finishes?
If you can send a live url to inspect the problematic behavior, this would be of great help in providing more to the point solution.
Another thing you could try is to move the AjaxLoadingPanel to the Default page and add it as LoadingPanelID to the updated control in RadAjaxManagerProxy settings.
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and share your thoughts.