So, after you log in, the zimbra server redirects your browser back to
http://hostname/zimbra/mail - which is failing, since you're not on port 80 - so, after you log in, you can manually add :8080 to the url:
http://hostname:8080/zimbra/mail - that should let you in.
That's a temporary solution, tho - if that works, add the proxyPort=8080 directive to /opt/zimbra/tomcat/conv/server.xml in the http connector section, and restart tomcat - that should get you going.