The provided information is not enough to isolate the root cause of the error you are facing. The problematic behaviour could ne be replicated locally in a sample application with ajaxified RadCalendar control. Therefore could you please share your page markup and related code behind so we could further research ion the issue?
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed