This happens while the browser  is idle for some time.
Just do a refresh  and restart the upload process it shud work.
If not, close the IE session and openup a new one and upload.

The error  you found should not have occured because
even when u are uploading  the browser  is not idle.

Ne ways just check if it i actually uploading the file
or not.

Chk this : http://support.microsoft.com/?kbid=828811&FR=1

But ideally, it SHOULD upload after a new session.

Add a form digest control to ur page. This should solve the problem