sssd.service ロギング: Could not start TLS encryption. unknown error

Issue

• sssd.service が systemctl status sssd.service で以下を表示して失敗します。

  sssd.service  sssd[be[domain]][PID]: Could not start TLS encryption. unknown error
  

• sssd ログで SSSD が失敗します。

  [be[example]] [sdap_uri_callback] (0x0400): Constructed uri 'ldaps://ldap.example.com:636'
  [be[example]] [sssd_async_socket_init_send] (0x4000): Using file descriptor [18] for the connection.
  [be[example]] [sssd_async_socket_init_send] (0x0400): Setting 6 seconds timeout for connecting
  [be[example]] [be_ptask_execute] (0x0400): Task [enumeration]: executing task, timeout 300 seconds
  [be[example]] [sdap_id_op_connect_step] (0x4000): waiting for connection to complete
  [be[example]] [sss_ldap_init_sys_connect_done] (0x0020): ldap_install_tls failed: [Connect error] [unknown error]
  [be[example]] [sss_ldap_init_state_destructor] (0x0400): calling ldap_unbind_ext for ldap:[0x2aa4d845090] sd:[18]
  [be[example]] [sss_ldap_init_state_destructor] (0x0400): closing socket [18]
  [be[example]] [sdap_sys_connect_done] (0x0020): sdap_async_connect_call request failed: [5]: Input/output error.
  [be[example]] [sdap_handle_release] (0x2000): Trace: sh[0x2aa4db584d0], connected[0], ops[(nil)], ldap[(nil)], destructor_lock[0], release_memory[0]
  [be[example]] [_be_fo_set_port_status] (0x8000): Setting status: PORT_NOT_WORKING. Called from: src/providers/ldap/sdap_async_connection.c: sdap_cli_connect_done: 1605