Hi,
This error is caused by a change in behavior of ASP.NET after a security update, see this thread for more background info.
However, I think you are not running the latest version of mojoPortal because I made some changes in content manager to solve this problem a few versions ago. If you upgrade to the latest version this problem should be fixed because we changed it so that if there are lots of pages we use a modal dialog to publish to avoid postback of too many form elements.
Hope that helps,
Joe