Perhaps ldap data is corrupt. Of course the following assumes that you have a good backup (as you mentioned).
Stop all running processes, and make a special effort to make sure slapd isn't running.
Export your ldap data, remove, and reimport per
http://www.zimbra.com/blog/archives/...ap_data_1.html
Note that slapcat and slapadd are located in /opt/zimbra/openldap-2.3.34/sbin
After you have imported, try running the installer again.