Error: Sys.InvalidOperationException: Handler was not added through the Sys.UI.DomEvent.addHandler method.

I hit this error after i upgraded my ajaxtoolkits in my ASP.NET project.

Error: Sys.InvalidOperationException: Handler was not added through the Sys.UI.DomEvent.addHandler method.

Finally i found the quick solution from this site by taking off the Enabled="false" for tabpanel

OR

Another better solution is get the latest version of ajaxtoolkits from here