What can cause Session Expired way before configured timeout value?

I am running a vaadin task I am developing, after posting a panel, a few seconds later I got this red Session Expired message in the browser window. But it can’t be truly session expired because the session-timeout is 3600 in web.xml and the task was not running nearly that long.
My question is that for problem like this, where should I go to look for debugging? This is vaadin 6 (sorry).
Please help. Thanks.