[global]
  server string = %h (Samba, Ubuntu)
  security = user
  #passdb backend = tdbsam
  #map to guest = never

  log file = /var/log/samba/log.%m
  log level = 5

[guest]
    path = /SMBMOUNT/HomePiFreigabe
    read only = no
    guest ok = yes

assdb backend = tdbsam

map to guest = never

[2019/02/21 20:48:05.953229,  3] ../source3/passdb/lookup_sid.c:1645(get_primary_group_sid)
  Forcing Primary Group to 'Domain Users' for pinedorus
[2019/02/21 20:48:05.953590,  2] ../libcli/auth/ntlm_check.c:424(ntlm_password_check)
  ntlm_password_check: NTLMv1 passwords NOT PERMITTED for user pinedorus
[2019/02/21 20:48:05.953678,  3] ../libcli/auth/ntlm_check.c:431(ntlm_password_check)
  ntlm_password_check: NEITHER LanMan nor NT password supplied for user pinedorus
[2019/02/21 20:48:05.954407,  2] ../source3/auth/auth.c:315(auth_check_ntlm_password)
  check_ntlm_password:  Authentication for user [pinedorus] -> [pinedorus] FAILED with error NT_STATUS_WRONG_PASSWORD

[2019/02/21 20:54:58.041053,  3] ../source3/auth/auth.c:181(auth_check_ntlm_password)
  check_ntlm_password:  mapped user is: [PISERVER][guest]@[GONDOLIN]
[2019/02/21 20:54:58.041368,  3] ../source3/auth/check_samsec.c:400(check_sam_security)
  check_sam_security: Couldn't find user 'guest' in passdb.
[2019/02/21 20:54:58.041470,  2] ../source3/auth/auth.c:315(auth_check_ntlm_password)
  check_ntlm_password:  Authentication for user [guest] -> [guest] FAILED with error NT_STATUS_NO_SUCH_USER

smbclient //PISERVER/guest --user=pinedorus
Enter pinedorus's password:
Domain=[WORKGROUP] OS=[Windows 6.1] Server=[Samba 4.5.16-Debian]
smb: >

check_ntlm_password: sam authentication for user [pinedorus] succeeded