Should be a nice quick fix, my zimbra.log file is zero bytes on my home server, the work one is about 2 meg, so I know it should be recording something!
I've searched and not found, but did see a mention of checking the permissions but that was a different issue. If it's relevent, the permissions on the file are:
Empty log: -rw-r--r-- 1 zimbra zimbra 0 Jul 16 13:45 /var/log/zimbra.log
Working log: -rw-r----- 1 zimbra root 2101134 Jul 18 09:22 /var/log/zimbra.log
So there is a difference, but I didnt want to just go changing it without knowing exactly what I'm changing.