Hello Pik Lee,
Take a look at this KB that explains possible reasons for the error you are getting - Intermittent Invalid Viewstate Error in ASP.NET Web pages
Also I see that you have some controls on this page and more specially a TextBox that implements IPostBackDataHandler and probably causes the issue.
All the best,
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.