I've setup a test install of zimbra. When I attempt to logon, I end up back at the logon screen. If I mistype the password I get told the username/password is incorrect. But when I do get it correct, firefox submits the page, and spins, then returns to the logon page.
Loging on to the admin interface works.
I'm doing external LDAP authentication, and that works.
I'm running apache on port 80, and proxyforwarding a virtual host to port 81, where zimbra is listening. I see the same behaviour if I point the browser directly at port 81.
I asked on the irc.freenode.net#zimbra, but we didn't come to any conclusions about what was going on.
Zimbra 4.0.2, installed with the debian packages.