SERVER_DOWN error appears when keystone accesses a backend LDAP server

Issue

• /var/log/containers/keystone/keystone.log shows SERVER_DOWN errors.

  ERROR keystone.common.wsgi [req-aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa - - - - -] {'info': "0000208D: NameErr: DSID-03100245, problem 2001 (NO_OBJECT), data 0, best match of:\n\t'DC=example,DC=com'\n", 'matched': 'DC=example,DC=com', 'desc': "Can't contact LDAP server"}: SERVER_DOWN: {'info': "0000208D: NameErr: DSID-03100245, problem 2001 (NO_OBJECT), data 0, best match of:\n\t'DC=example,DC=com'\n", 'matched': 'DC=example,DC=com', 'desc': "Can't contact LDAP server"}
  
      or
  
  ERROR keystone.common.wsgi [req-aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaa - default default] {'desc': "Can't contact LDAP server"}: SERVER_DOWN: {'desc': "Can't contact LDAP server"}
  

• This error leads 500 error or 503 error when accessing OpenStack API

  (overcloud)[stack@director ~]$ openstack server list
  An unexpected error prevented the server from fulfilling your request. (HTTP 500) (Request-ID: req-aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa)
  

• However, no problems are observed in the LDAP server side nor network side.