Nimm das List mal aus dem Connect-Event raus und versuchs mal so
Code:
begin
  IdFTP1.Host := editFTPHost.Text;
  IdFTP1.UserName := editFTPUsername.Text;
  IdFTP1.Password := editFTPPassword.Text;
  IdFTP1.AutoLogin := True;
  try
    IdFTP1.Connect;
    IdFTP1.List(DirectoryListBox1.Items,'*.*',FALSE);
  except
    on E: Exception do ShowMessage(E.Message);
  end;
end;