I had similar issues, phoenix has a good suggestion, but for me, I didn't have any bad tables..
check that you're /var/log/zimbra.log has server statistics data:
grep STATUS /var/log/zimbra.log
if you don't see any STATUS logs on your server(s), make sure you've got syslogd setup, and your zimbra cron for zmstatuslog is setup.
-Jessie |