LDAP connects but fails to return entries

I am developing a plugin. In local environment if works fine. On the real server it fails, returns 0 entries. The method is called from a template:

ret = "{{craft.nastupy.hledanizamestancu('"+retezec+"', 0) }}"//hledanizamestancu
alert("ret " + ret)

It resides in /variables, and goes like so:

public function hledanizamestancu($retezec) {
  //error_reporting(0);
  $ldap_dn = "intentionally left blank";
  $ldap_password = "so is this";
  $ldap_conn = ldap_connect("and this", 389);
  //print($ldap_conn);  
  ldap_set_option($ldap_conn, LDAP_OPT_PROTOCOL_VERSION, 3);
  ldap_set_option($ldap_conn, LDAP_OPT_DEBUG_LEVEL, 7);
  ldap_set_option($ldap_conn, LDAP_OPT_REFERRALS, true);
  
  if (@ldap_bind($ldap_conn, $ldap_dn, $ldap_password)) {
      echo ldap_error($ldap_conn);
      $dn = "blank once again";
      $filter = "(displayName=*".$retezec."*)";
      $justthese = array("displayName", "email");
      $result = ldap_search($ldap_conn, $dn, $filter, $justthese);
      $entries = ldap_get_entries($ldap_conn, $result);
      $vystup = "";
      foreach ($entries as $e) {
          print("e ".$e);
          if (isset($e['email'][0]) && isset($e['displayname'][0])) {
              $vystup .= $e['displayname'][0].",";
          }
      }
      print("vystup " .$vystup. " konec");
  } else {
    print("I should never ever see this.");
  }
}

I tried both displayname of displayName in the foreach loop, but they fail both. I also tried AJAX directly from JS. Same story. It works in local environment just fine but fails on the server.
Thank you.