Hello,
Does it happen always or time to time?
It could happen if somebody do a postback before the whole viewstate of page is loaded, e.g. if viewstate is too long and/or the connection is slow and user submit some button before the page is completely loaded. Unfortunately, this is behavior of .NET platform, it is not caused by CMS.
Other possibile solution:
http://community.discountasp.net/showthread.php?t=2719Best regards,
Helena Grulichova