cioè, il tuo file control_log.php è così?
codice:
<?php
session_start();

$userid = $_POST['userid'];
$password_user = $_POST['password_user'];

$sql = "SELECT * FROM tabella where userid = '$userid' AND password_user = '$password_user' and autorizza = 'Y'";
$result = mysql_db_query($database,$sql,$db) or die("Errore ".mysql_error());

$num_rows = mysql_num_rows($result);
if ( $num_rows != 0 ) {
  // trovato
  $row = mysql_fetch_array($result);
  $_SESSION['loggato'] = true;
  $_SESSION['id_ute'] = $row['id_ute'];
  $_SESSION['userid'] = $row['userid'];
  $_SESSION['nome'] = $row['nome'];
  $_SESSION['password_user'] = $row['password_user'];
  $_SESSION['livello'] = $row['livello'];
  $_SESSION['email'] = $row['email'];
  $_SESSION['accessi'] = $row['acessi'];
  
  header ("Location: office/split.php");
} 
else {
  // non trovato
  header ("Location: index.php?go=errore"); 
}
?>
e split di conseguenza
codice:
<?php
session_start();
?>

<html>
<head>
</head>
<title>AREA RISERVATA </title>

<?php

if (( $_SESSION['loggato'] == true ) and ( $_SESSION['livello'] == "webmaster"))


{

echo("<frameset cols=280,* frameborder=no border=0 framespacing=0>");
echo("<frame src=admin_office/menu.php name=menu scrolling=auto noresize>");
echo("<frameset rows=100,* frameborder=no border=0 framespacing=0>");
echo("<frame src=admin_office/testa.php name=testa scrolling=auto noresize>");
echo("<frame src=admin_office/corpo.php name=corpo scrolling=auto noresize>");
echo("</frameset>");
echo("<noframes></noframes></frameset>");


}

if (( $_SESSION['loggato'] == true ) and ( $_SESSION['livello'] == "amministratore"))


{

echo("<frameset cols=280,* frameborder=no border=0 framespacing=0>");
echo("<frame src=admin_office/menu.php name=menu scrolling=auto noresize>"); //correzione qui
echo("<frameset rows=100,* frameborder=no border=0 framespacing=0>");
echo("<frame src=admin_office/testa.php name=testa scrolling=auto noresize>");
echo("<frame src=admin_office/corpo.php name=corpo scrolling=auto noresize>");
echo("</frameset>");
echo("<noframes></noframes></frameset>");


}

if (( $_SESSION['loggato'] == true ) and ( $_SESSION['livello'] == "utente"))



{

echo("<frameset cols=280,* frameborder=no border=0 framespacing=0>");
echo("<frame src=user/menu.php name=menu scrolling=auto noresize>");
echo("<frame src=user/corpo.php name=corpo scrolling=auto noresize>");
echo("</frameset><noframes></noframes>");
echo("</frameset>");

}

if (( $_SESSION['loggato'] == true ) and ( $_SESSION['livello'] == "segretaria"))


{

echo("<frameset cols=280,* frameborder=no border=0 framespacing=0>");
echo("<frame src=admin_office/menu_segre.php name=menu_segra scrolling=auto noresize>");
echo("<frame src=admin_office/corpo_segre.php name=corpo_segre scrolling=auto noresize>");
echo("</frameset><noframes></noframes>");
echo("</frameset>");

}


else {

echo ("

");
echo ("<div align=center>");
echo ("





");
echo ("<font face=Arial style=font-size:18px color=#F90606>");
echo ("Per accedere al pannello devi loggarti");
echo ("</font>");
echo ("</div>");
}


?>

</html>