I have a web application developed in ASP.net & C#. I also use Telerik ASP.NET AJAX for web UI. Application throws an exception (System.Web.UI.ViewStateException: Inva
There are a host of reasons why this could occur, a common one being worker process recycling/Application Pool Recycling on your IIS server.
Have a look at the below articles to try and debug why exactly it is occuring with you:
http://support.microsoft.com/kb/555353
More specifically to determine the error have a look at the "Determine exactly what exception occurs when you receive the error message" part in the below link:
http://support.microsoft.com/kb/829743