Not sure if you're in the same boat as I am, but I posted concerning a similar issue last week. It seems that when I upgraded to 4.5.6 a Zimbra MySQL table became corrupted. Everything else besides the Zimbra statistics seems to be working fine, but my attempts to repair the table have failed. If you are having the same problem, and you come up with a solution please post back.
For your reference here's the link to
my post, with details of where I found the info about the corrupt table, and what has been suggested to fix it.