Please ensure that the ajax settings for the RadTabStrip and RadMultiPage controls are added correctly as it is shown in the following help topic
Also please note that the RadAjaxLoadingPanel control should have a Skin property set in order to appear correctly.
Give this a try and let me know if it helps.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed