[Zope] empty session with virtual hosting

I have a Zope 2.9.0 installation under Solaris. When I access the Zope site directly (port 8080) the session has the keys I expect, but when I try to do the same using the virtual host (port 80 has apache that rewrites to the zope installation) the session is empty. Do you have any hint about this behaviour ? thank you in advance! Luca