ช่วยหน่อยครับ ติดปัญหาเกี่ยวกับการเปิด pdf บน server จริงครับ (HTTP error 500)

<?php
ob_start();
session_start();
require_once('../../mpdf/mpdf.php');
?>
<meta charset="UTF-8">
<?php
require_once('../../connect.php');
$query = "SELECT * FROM customer order by cus_id asc";
$result = mysqli_query($con, $query);
?>
<?php
$sql_emp = "SELECT * FROM employee WHERE emp_id='" . $_SESSION['emp_id'] . "'";
$result_emp = mysqli_query($con, $sql_emp);
$row_emp = mysqli_fetch_array($result_emp);
?>
<br>
<div align="right"><?php echo date("d-m-Y H:i:s"); ?> : <?php echo $row_emp['emp_name']; ?> 

<?php echo $row_emp['emp_surname']; ?>&nbsp;</div>
<h3 align="center">รายงานชื่อลูกค้า</h3>

<table width="1000" border="1" cellpadding="0"  cellspacing="0" align="center">
    <thead>
        <tr>
            <th style="text-align: center">ลำดับที่</th>
            <th style="text-align: center">ชื่อ-นามสกุล</th>
            <th style="text-align: center">อีเมล</th>
            <th style="text-align: center">เบอร์โทรศัพท์</th>
            <th style="text-align: center">บ้านเลขที่</th>
            <th style="text-align: center">ตำบล</th>
            <th style="text-align: center">อำเภอ</th>
            <th style="text-align: center">จังหวัด</th>
            <th style="text-align: center">รหัสไปรษณีย์</th>
        </tr>
    </thead>

    <?php while ($row = mysqli_fetch_array($result)) {
        ?>
        <tr>
            <td align="center"><?php echo $i += 1; ?> </td>
            <td  width="250"><?php echo $row['cus_perfix']; ?> <?php echo $row['cus_name']; ?>  
<?php echo $row['cus_surname']; ?></td>
            <td><?php echo $row['cus_email']; ?></td>
            <td align="center"><?php echo $row['cus_number']; ?></td>
            <td align="center"><?php echo $row['cus_address']; ?></td>
            <td align="center"><?php echo $row['cus_district']; ?></td>
            <td align="center"><?php echo $row['cus_prefecture']; ?></td>
            <td align="center"><?php echo $row['cus_province']; ?></td>
            <td align="center"><?php echo $row['cus_zip_code']; ?></td>
        </tr>

        <?php
    }
    ?>
</table>
<?php mysqli_close($con); ?>
<?php
$html = ob_get_contents();
ob_end_clean();
$pdf = new mPDF('th', 'A4-L', '', '', 0, 0, 0, 0, 0, 0);
$pdf->SetAutoFont();
$pdf->SetdisplayMode('fullpage');
$pdf->WriteHTML($html);
$pdf->SetY(-15);
$pdf->AliasNbPages('{page}');
$pdf->Cell(0,10,'Page '.$pdf->PageNo()." / {page}",0,0,'R');
$pdf->AliasNbPages('{page}');
$pdf->Output('report_customer.pdf', 'I');
?>