Check the permission of the /opt/zimbra/zmstat directory:

# ls -la /opt/zimbra/zmstat
drwxr-xr-x   2 root   root       4096 Apr 26 00:00 2010-04-25
drwxr-xr-x   2 root   root       4096 Apr 27 00:00 2010-04-26
drwxr-xr-x   2 root   root       4096 Apr 28 00:00 2010-04-27
drwxr-xr-x   2 root   root       4096 Apr 29 00:00 2010-04-28
drwxr-xr-x   2 root   root       4096 Apr 30 00:00 2010-04-29
drwxr-xr-x   2 root   root       4096 May  1 00:00 2010-04-30
drwxr-xr-x   2 root   root       4096 May  2 00:00 2010-05-01

–> if any directory has root:root permission:

–> then do;

# su - zimbra
[zimbra@hostname ~]$ zmstatctl stop
[zimbra@hostname ~]$ exit

as root;

# cd /opt/zimbra/zmstat
# chown -R zimbra:zimbra *
# su - zimbra
[zimbra@hostname ~]$ zmstatctl start
[zimbra@hostname ~]$ zmstatctl status

Solution found on Zimbra forums –> http://www.zimbra.com/forums/installation/39421-zmstatctl-doesnt-start-2.html