I recently upgraded from 4.5.6 to 4.5.11 and the daily reports stopped. I have read many thread on the board about this; however, none were specific to correct this on my system.
I found one post about checking the zmstat.out for errors. I am getting some errors in that log, these lines are repeated often:
No iostat installed on this host at /opt/zimbra/libexec/zmstat-io line 27.
Warning: Not possible to monitor process stats
07/27/2008 00:37:00: ERROR: No such process: '8600'
07/27/2008 00:37:00: ERROR: No such process: '8601'
This is running on a RHEL4.x Server.