Telerik says you should either use RadAjaxManager OR RadAjaxPanel, but not both.
The issue is, if you are using ASP.net web forms and have RadAjaxManager in the master page, it is not exactly ideal to add code to ajaxify your controls in every page as such:
The easy fix should be to replace RadAjaxPanel with <asp:panel> and ajaxify the panel through RadAjaxManager.
The second issue using RadAjaxManager and RadAjaxPanel together is that
RadAjaxManager.Getcurrent(Page).IsAjaxRequest almost always returns false
I have done my work to compare the HTML code, it is exactly the same as it would be if you used an asp:update panel, so it is simply the logic in the code behind. In which case, I find the asp.net ScriptManager to be sufficient