I've just installed SmartPart for SharePoint from here
and created a sample user control which has RadTextBox which is ajaxified via RadAjaxManager. Everything worked without issues on my end.
You can find the usercontrol and web.config file which I am using attached to this post. Have in mind that in the /bin folder you should place Telerik.Web.UI.dll and Telerik.Charting.dll assemblies.
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