Answer
Apps9 needs to be running at least UPD14, which introduced a solution enabling LDAPS, LDAP over SSL, using the default LDAPS port 636, specifically for AD Synchronization. Environments below UPD14 don't have this LDAPS-enablement solution and would need to update to at least that level, or investigate whether the fix is available as a standalone single patch, to properly support LDAP channel binding and signing enforcement on the Domain Controller side.