A conexão do Samba falha no OS X

A conexão do Samba falha no OS X

O servidor está executando Ubuntu 12.04 e Samba 3.6.3. Os usuários do Windows podem montar seus compartilhamentos sem problemas, usando o caminho \\my_server.com\usernamee a conta \username.

No entanto, quando tento conectar-me ao cliente GUI nativo do OS X 10.10.5, a autenticação falha. O registro fica assim:

2015/09/29 15:47:23.536252,  3] smbd/sesssetup.c:1107(reply_sesssetup_and_X_spnego)
  NativeOS=[Mac OS X 10.10] NativeLanMan=[SMBFS 3.0.2] PrimaryDomain=[]
[2015/09/29 15:47:23.536318,  3] ../libcli/auth/ntlmssp_server.c:348(ntlmssp_server_preauth)
  Got user=[OSX_user] domain=[MY_SERVER] workstation=[workstation] len1=24 len2=260
[2015/09/29 15:47:23.536415,  3] auth/auth.c:219(check_ntlm_password)
  check_ntlm_password:  Checking password for unmapped user [MY_SERVER]\[OSX_user]@[workstation] with the new password interface
[2015/09/29 15:47:23.536469,  3] auth/auth.c:222(check_ntlm_password)
  check_ntlm_password:  mapped user is: [MY_SERVER]\[OSX_user]@[workstation]
[2015/09/29 15:47:23.536638,  3] passdb/lookup_sid.c:1754(get_primary_group_sid)
  Forcing Primary Group to 'Domain Users' for OSX_user
[2015/09/29 15:47:23.536825,  3] ../libcli/auth/ntlm_check.c:392(ntlm_password_check)
  ntlm_password_check: NTLMv2 password check failed
[2015/09/29 15:47:23.536901,  3] ../libcli/auth/ntlm_check.c:437(ntlm_password_check)
  ntlm_password_check: Lanman passwords NOT PERMITTED for user OSX_user
[2015/09/29 15:47:23.537053,  3] ../libcli/auth/ntlm_check.c:585(ntlm_password_check)
  ntlm_password_check: LM password, NT MD4 password in LM field and LMv2 failed for user OSX_user
[2015/09/29 15:47:23.537268,  2] auth/auth.c:319(check_ntlm_password)
   check_ntlm_password:  Authentication for user [OSX_user] -> [OSX_user] FAILED with error NT_STATUS_WRONG_PASSWORD
[2015/09/29 15:47:23.537346,  3] smbd/error.c:81(error_packet_set)
   error packet at smbd/sesssetup.c(124) cmd=115 (SMBsesssetupX) NT_STATUS_LOGON_FAILURE

Alguma idéia de por que isso acontece? Preciso habilitar o NTLM para usuários do OS X?

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