I discover something interesting.
There is a file which log each try domain joining
c:\windows\debug\NetSetup.LOG
here is this file with the two tests (legacy and new client)
NetSetup.LOG with the NEW client (which failed)
10/02 10:18:24 -----------------------------------------------------------------
10/02 10:18:24 NetpDoDomainJoin
10/02 10:18:24 NetpMachineValidToJoin: 'gim-127-13'
10/02 10:18:24 NetpGetLsaPrimaryDomain: status: 0x0
10/02 10:18:24 NetpMachineValidToJoin: status: 0x0
10/02 10:18:24 NetpJoinDomain
10/02 10:18:24 Machine: gim-127-13
10/02 10:18:24 Domain: samba_domain
10/02 10:18:24 MachineAccountOU:
10/02 10:18:24 Account: samba_domain\admin_samba
10/02 10:18:24 Options: 0x3
10/02 10:18:24 OS Version: 5.1
10/02 10:18:24 Build number: 2600
10/02 10:18:24 ServicePack: Service Pack 3
10/02 10:18:24 NetpValidateName: checking to see if 'samba_domain' is valid as type 3 name
10/02 10:18:24 NetpValidateName: 'samba_domain' is not a valid Dns domain name: 0x2554
10/02 10:18:25 NetpCheckDomainNameIsValid [ Exists ] for 'samba_domain' returned 0x0
10/02 10:18:25 NetpValidateName: name 'samba_domain' is valid for type 3
10/02 10:18:25 NetpDsGetDcName: trying to find DC in domain 'samba_domain', flags: 0x1020
10/02 10:18:25 NetpDsGetDcName: found DC '\\SAMBA' in the specified domain
10/02 10:18:25 NetpJoinDomain: status of connecting to dc '\\SAMBA': 0x0
10/02 10:18:25 NetpJoinDomain: OU is specified but couldn't get NT5 DC
10/02 10:18:25 NetpJoinDomain: status of disconnecting from '\\SAMBA': 0x0
10/02 10:18:25 NetpDoDomainJoin: status: 0x54b
10/02 10:19:26 -----------------------------------------------------------------
NetSetup.LOG with the LEGACY client (which works fine)
10/02 10:50:12 -----------------------------------------------------------------
10/02 10:50:12 NetpDoDomainJoin
10/02 10:50:12 NetpMachineValidToJoin: 'gim-127-13'
10/02 10:50:12 NetpGetLsaPrimaryDomain: status: 0x0
10/02 10:50:12 NetpMachineValidToJoin: status: 0x0
10/02 10:50:12 NetpJoinDomain
10/02 10:50:12 Machine: gim-127-13
10/02 10:50:12 Domain: samba_domain
10/02 10:50:12 MachineAccountOU: (NULL)
10/02 10:50:12 Account: samba_domain\admin_samba
10/02 10:50:12 Options: 0x3
10/02 10:50:12 OS Version: 5.1
10/02 10:50:12 Build number: 2600
10/02 10:50:12 ServicePack: Service Pack 3
10/02 10:50:12 NetpValidateName: checking to see if 'samba_domain' is valid as type 3 name
10/02 10:50:12 NetpValidateName: 'samba_domain' is not a valid Dns domain name: 0x2554
10/02 10:50:12 NetpCheckDomainNameIsValid [ Exists ] for 'samba_domain' returned 0x0
10/02 10:50:12 NetpValidateName: name 'samba_domain' is valid for type 3
10/02 10:50:12 NetpDsGetDcName: trying to find DC in domain 'samba_domain', flags: 0x1020
10/02 10:50:20 NetpDsGetDcName: found DC '\\SAMBA' in the specified domain
10/02 10:50:20 NetpJoinDomain: status of connecting to dc '\\SAMBA': 0x0
10/02 10:50:20 NetpGetLsaPrimaryDomain: status: 0x0
10/02 10:50:20 NetpGetNt4RefusePasswordChangeStatus: trying to read from '\\SAMBA'
10/02 10:50:20 NetpGetNt4RefusePasswordChangeStatus: RefusePasswordChange == 0
10/02 10:50:20 NetpLsaOpenSecret: status: 0xc0000034
10/02 10:50:21 NetpManageMachineAccountWithSid: NetUserAdd on '\\SAMBA' for 'GIM-127-13$' failed: 0x8b0
10/02 10:50:21 NetpManageMachineAccountWithSid: status of attempting to set password on '\\SAMBA' for 'GIM-127-13$': 0x0
10/02 10:50:21 NetpJoinDomain: status of creating account: 0x0
10/02 10:50:21 NetpGetLsaPrimaryDomain: status: 0x0
10/02 10:50:21 NetpSetLsaPrimaryDomain: for 'SAMBA_DOMAIN' status: 0x0
10/02 10:50:21 NetpJoinDomain: status of setting LSA pri. domain: 0x0
10/02 10:50:21 NetpJoinDomain: status of managing local groups: 0x0
10/02 10:50:21 NetpJoinDomain: status of setting netlogon cache: 0x0
10/02 10:50:22 NetpJoinDomain: status of clearing ComputerNamePhysicalDnsDomain: 0x0
10/02 10:50:22 NetpUpdateW32timeConfig: 0x0
10/02 10:50:22 NetpJoinDomain: status of disconnecting from '\\SAMBA': 0x0
10/02 10:50:22 NetpDoDomainJoin: status: 0x0
10/02 10:53:12 -----------------------------------------------------------------
10/02 10:53:12 NetpDoDomainJoin
10/02 10:53:12 NetpMachineValidToJoin: 'gim-127-13'
10/02 10:53:12 NetpGetLsaPrimaryDomain: status: 0x0
10/02 10:53:12 NetpMachineValidToJoin: the specified machine is already joined to 'SAMBA_DOMAIN'!
10/02 10:53:12 NetpMachineValidToJoin: status: 0xa83
10/02 10:53:12 NetpDoDomainJoin: status: 0xa83
May be it could help to find the problem