Sheesh, I feel stupid about this one. When I first installed on Debian, I had several problems that I worked through. I kept notes so that Zimbra folks could fix things and so other Debian users could avoid some pitfalls.
I saw that all my problems were already noted by others. EXCEPT, the redo.log problem. I forgot to write it down and today it bit me again.
I installed webmin and after rebooting the zimbra machine has errors! Mailbox not running. After chasing around in the logs, I found this error in /opt/zimbra/logs/zimbra.log. (BTW, why is this zimbra.log different from /var/log/zimbra.log?)
java.io.FileNotFoundException:/opt/zimbra/redolog/redo.log (Permission denied)
The file is owned by root.root with permissions of rw-r-----(640). Hmmm, I remembered this exact same thing happening during my first install. Now it is back after a reboot. I changed the group ownership to zimbra and changed the mods to -rw-rw--- (660). Restarted zimbra and VOILA, everything works again.
Upon rebooting, the file is changed back to root.root rw-r-----(640). So, it is necessary to change the file permissions after each reboot.