Okay, so what you could try is extracting your tarball somewhere else on the server. Switch into the zimbra user. Change to the directory where your extracted backup is and then run
/opt/zimbra/openldap/sbin/slapcat -F /opt/zimbra/data/ldap/config -b "" -l /opt/zimbra/data/ldap/ldap.bak.
You may wish to backup the ldap.bak first or even move it to a subdirectory out of the way. Hopefully this will generate a good LDAP backup using the V5 binaries.
Once that is done, if it works, copy the ldap.bak to /opt/zimbra/data/ldap and try the upgrade again.