Hello Timyr,
I am not sure why the problem occurs, but if you think the problem could be the default ScriptManager of DNN, you can replace it with RadScriptManager. Here is a help topic on the matter -
http://www.telerik.com/help/aspnet-ajax/dnn-replace-scriptmanager-radscriptmanager.html
All the best,
Peter
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.