ViewState exception

Hi, We are using Daypilot 5.6.1820. Sometimes when performing an action on the calendar control (e.g. moving, resizing events) we encounter the following exception: System.Web.HttpException: The state information is invalid for this page and might be corrupted. ---> System.Web.UI.ViewStateException: Invalid viewstate. I was just wondering if this is an issue you have encountered before? The reason I am posting it here is that the issue is only occurring on pages with the calendar control and only if we are interacting with the control in some way. Thanks, Kieran