I'd recommend both looking at the LDAP Authentication wiki page (which includes how to setup logging) at:
LDAP Authentication - Zimbra :: Wiki
Also, when troubleshooting LDAP authentication issues I find it much easier to use a packet capture program (either on the LDAP server or client). Wireshark (formerly Ethereal) has helped a lot for me in the past - even on systems which are MUCH more of a PITA than Zimbra's (relatively painfree) LDAP authentication.
State University of New York at New Paltz