[Zope] Server does not answer

I have a Zope session that runs a Plone one. Navigating on the site is fine. But when one try to connect, the navigator sends a request to the Zope server. It sends back an acknowledgment : the navigator informs the web site is found. And then the navigator is waiting for data that never comes ? I have no error logged on my Zope instance. It is a FreeBSD 5 server that runs a Zope/(unreleased version, python 2.3.4 with ZServer/1.1 (Plone version : 2.0.3). What can be the problem ? Regards. Sebastien