It probably choked the first time because you had LDAP data that wouldn't migrate, due to using something like the domain disclaimer or posix/samba, and you didn't follow the steps to prepare your server for upgrade in that scenario. Thus the second time you ran the upgrade, you had no LDAP data at all, and it "succeeded". I would suggest looking at /opt/zimbra/data/ldap directory and see if you find your old pre-upgrade LDAP database in there. You'll still need to fix whatever issue it ran into when upgrading before you can import it.
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