have you confirmed problem is gone when you comment out CKEditor related code?

no, still remains.

I had to go and fix all unquoted values in your input plus you didnt close much of your html tags
<input ..../> was missing a lot. Take a look of rough fix below:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Admin</title>
<link href="admin-style.css" rel="stylesheet" type="text/css" />
<link rel="stylesheet" href="includes/navstyleadmin.css" />
<script type="text/javascript" src="includes/jquery-1.4.2.min.js"></script>
<script type="text/javascript" src="includes/superfish.js"></script>
<script type="text/javascript" src="ckeditor/ckeditor.js"></script>
<link href="ckeditor/content.css" rel="stylesheet" type="text/css"/>
<script type="text/javascript">
jQuery(function(){
	jQuery('ul.menu').superfish({
		animation: {opacity:'show',height:'show'},
		speed: 'slow', 
		autoArrows: true
	});
});
</script>

<style type="text/css">
<!--
.style1 {font-size: 12px}
-->
</style>
</head>

<body>
<div id="header">
      <p><a href="#">Home </a>| <a href="index.php">Logout </a>| <a href="admin.php">Admin Panel</a></p>
      <p>&nbsp;</p>
      <p>&nbsp;</p>
      <p>&nbsp;</p>
    </div>
    
    <div id="search">
      
      <form action="search.php" method="post" name="login" target="_self" id="login" style="style.css" title="search">
      	<br>
        <label>search :
        <input type="text" name="search" id="search2" />
</label>
        <br />
                    
      </form> 		
	</div>
    
    <div id="menu">
            
<?php //include ("includes/navigation_admin.php"); ?>
<?php //include ("includes/koneksi.php"); ?>        
	 </div>     
        

<?php


// DATA BARU

//if ($_REQUEST[mode]=='edit')
//{extract ($_REQUEST);
//do something  

//echo $_REQUEST['id'];
//will give us the valid id from the previous page.
//}

if (empty($_GET['id']))
{
$nama = '';
$kategori = '';
$harga = '';
$deskripsi = '';
}


$nama = isset($_POST['nama']) ? $_POST['nama'] : '';	  
$kategori = isset($_POST['kategori']) ? $_POST['kategori'] : '';
$harga = isset($_POST['harga']) ? $_POST['harga'] : '';	  
$gambar = isset($_POST['gambar']) ? $_POST['gambar'] : '';	  

// SETELAH TOMBOL SIMPAN DI TEKAN

if (isset($_GET['save'])){
		$nama = $_GET['nama'];
		$kategori = $_GET['kategori'];
		$harga = $_GET['harga'];
		$gambar = $_GET['gambar'];
		
		/*if (empty($_GET['id'])){
			$result = mysql_query("INSERT INTO produk(nama_produk, harga, gambar) VALUES('".$nama."','".$kategori."','".$gambar."')");
			}
		else{
			$result = mysql_query("UPDATE produk SET nama_produk='".$nama_produk."', harga='".$harga."', deskripsi='".$deskripsi."', gambar='".$gambar."' WHERE id=".$_GET['id']);
		$result = mysql_query($sqlstr) or die(mysql_error());
		
		//Jika mode edit, maka tidak akan dikirimkan konfirmasi kepada subscriber
		//if (empty($_REQUEST['id']))	kirimEmail($idKategori, $judul, $news);
		$confirmation = ($result) ? "Data telah tersimpan." : "Gagal menyimpan data.";	
		}*/
	}

?>
        
<h2><center>Product Preview</center></h2>
<p></p>

<?php 

if(!empty($_GET['id']))
	{
	/*$result = mysql_query("SELECT * FROM produk AS p, kategori AS k WHERE id_produk='".mysql_real_escape_string($_GET['id'])."'AND 			    p.id_kategori = k.id_kategori") or die(mysql_error());
    $data = mysql_fetch_array($result);*/
    }
else
	{
	$data['nama_produk'] = '';
	$data['nama_kategori'] = '';
	$data['harga'] = '';
	$data['deskripsi'] = '';
	$data['gambar'] = '';
	}		 

?>

<form method = "POST" enctype="multipart/formdata" action = "<?php $_SERVER['PHP_SELF']; ?>"> 
<table border="0" cellpadding="2">

        <tr>
        	<td>Nama</td>
        	<td><input name="nama" size="20px" type="text" value ="<?php echo $data['nama_produk']; ?>" />
        </tr>
        <tr>
        	<td>Kategori</td>
         	<td><select name="kategori"><option value="<?php echo $data['nama_kategori']; ?>" selected><?php echo $data['nama_kategori']; ?></option>
             			<?php 		
						
							//$resultCOMBO = mysql_query("SELECT nama_kategori FROM kategori") or die(mysql_error());			
							//while($dataCOMBO = mysql_fetch_array($resultCOMBO)){
							         //echo "<option value={$dataCOMBO['nama_kategori']}>{$dataCOMBO['nama_kategori']}</option>";	
						//}?>	
                </select>              
            </td>

       </tr>
        <tr>
        	<td>Harga</td>
            <td><input name="harga" type="text" value = "<?php echo $data['harga']; ?>" /></td>
        </tr>
        <tr>
        	<td>Deskripsi</td>
            <td>
                    <textarea  id="editor" name="deskripsi" cols="30" rows="15" value="<?php echo $data['deskripsi']; ?>"></textarea>
                    <script type="text/javascript">
					    CKEDITOR.replace('editor');
			        </script>	
			     </td>
        </tr>
        <tr>
        	<td>Gambar</td>
			<td><img src="foto_produk/<?php echo $data['gambar'];?>" width="30%" height="30%" /></td>            
        </tr>	
		<tr>
        	<td>Ganti Gambar</td>
            <td>
            <input name="uploaded_file" type="file" value="Browse" /></td>
            </form>
        </tr>	
        <tr>
        	<td></td>
            <td><br /><center><input name="save" type="button" value="Simpan" /></center></td>
        </tr>
        
	</tr>
    
 
   <?php // include('includes\footer.php'); ?>

    
<p>&nbsp;</p>
</body>
</html>
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