Почему JQWidget показывает внутреннюю ошибку сервера 500 при фильтрации?

Я использовал JQwidgets для отображения данных из базы данных mysql с фильтрами, но всякий раз, когда я пытаюсь отфильтровать данные, выдается внутренняя ошибка сервера. Просто интересно, в чем может быть причина? Это из-за объединения таблиц? или по любой другой причине:

data.php

<?php
// Include the connect.php file
include ('connect.php');
// Connect to the database
// connection String
$mysqli = new mysqli($hostname, $username, $password, $database);
/* check connection */
if (mysqli_connect_errno())
{
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}
$pagenum = $_GET['pagenum'];
$pagesize = $_GET['pagesize'];
$start = $pagenum * $pagesize;
$query = "SELECT SQL_CALC_FOUND_ROWS cc.name AS 'Category', c.fullname
AS 'Course', u.firstname AS 'Name' , u.lastname AS 'EmployeeID',
CASE WHEN gi.itemtype = 'Course'
THEN c.fullname + ' Course Total'
ELSE gi.itemname
END AS 'Activity', CASE WHEN gg.finalgrade IN (0.00, NULL) THEN 'Absent'
AND AS 'State' FROM mdl_course AS c JOIN mdl_context AS ctx ON c.id = ctx.instanceid
JOIN mdl_role_assignments AS ra ON ra.contextid = ctx.id
JOIN mdl_user AS u ON u.id = ra.userid
JOIN mdl_grade_grades AS gg ON gg.userid = u.id
JOIN mdl_grade_items AS gi ON gi.id = gg.itemid AND gi.itemmodule = 'attendance' AND gi.courseid = c.id
JOIN mdl_course_categories AS cc ON cc.id = c.category LIMIT ?, ?";
$result = $mysqli->prepare($query);
$result->bind_param('ii', $start, $pagesize);
// filter data.
if (isset($_GET['filterscount']))
{
$filterscount = $_GET['filterscount'];
if ($filterscount > 0)
{
$where = " WHERE (";
$tmpdatafield = "";
$tmpfilteroperator = "";
$valuesPrep = "";
$values = [];
for ($i = 0; $i < $filterscount; $i++)
{
// get the filter's value.
$filtervalue = $_GET["filtervalue" . $i];
// get the filter's condition.
$filtercondition = $_GET["filtercondition" . $i];
// get the filter's column.
$filterdatafield = $_GET["filterdatafield" . $i];
// get the filter's operator.
$filteroperator = $_GET["filteroperator" . $i];
if ($tmpdatafield == "")
{
$tmpdatafield = $filterdatafield;
}
else if ($tmpdatafield <> $filterdatafield)
{
$where.= ")AND(";
}
else if ($tmpdatafield == $filterdatafield)
{
if ($tmpfilteroperator == 0)
{
$where.= " AND ";
}
else $where.= " OR ";
}
// build the "WHERE" clause depending on the filter's condition, value and datafield.
switch ($filtercondition)
{
case "CONTAINS":
$condition = " LIKE ";
$value = "%{$filtervalue}%";
break;

case "DOES_NOT_CONTAIN":
$condition = " NOT LIKE ";
$value = "%{$filtervalue}%";
break;

case "EQUAL":
$condition = " = ";
$value = $filtervalue;
break;

case "NOT_EQUAL":
$condition = " <> ";
$value = $filtervalue;
break;

case "STARTS_WITH":
$condition = " LIKE ";
$value = "{$filtervalue}%";
break;

case "ENDS_WITH":
$condition = " LIKE ";
$value = "%{$filtervalue}";
break;

case "NULL":
$condition = " IS NULL ";
$value = "%{$filtervalue}%";
break;

case "NOT_NULL":
$condition = " IS NOT NULL ";
$value = "%{$filtervalue}%";
break;
}
$where.= " " . $filterdatafield . $condition . "? ";
$valuesPrep = $valuesPrep . "s";
$values[] = & $value;
if ($i == $filterscount - 1)
{
$where.= ")";
}
$tmpfilteroperator = $filteroperator;
$tmpdatafield = $filterdatafield;
}
$valuesPrep = $valuesPrep . "ii";
$values[] = & $start;
$values[] = & $pagesize;
// build the query.
$query = "SELECT SQL_CALC_FOUND_ROWS cc.name AS 'Category', c.fullname AS 'Course', u.firstname AS 'Name' , u.lastname AS 'EmployeeID',
CASE WHEN gi.itemtype = 'Course'
THEN c.fullname + ' Course Total'
ELSE gi.itemname
END AS 'Activity', CASE WHEN gg.finalgrade IN (0.00, NULL) THEN 'Absent' END AS 'State' FROM mdl_course AS c JOIN mdl_context AS ctx ON c.id = ctx.instanceid
JOIN mdl_role_assignments AS ra ON ra.contextid = ctx.id
JOIN mdl_user AS u ON u.id = ra.userid
JOIN mdl_grade_grades AS gg ON gg.userid = u.id
JOIN mdl_grade_items AS gi ON gi.id = gg.itemid AND gi.itemmodule = 'attendance' AND gi.courseid = c.id
JOIN mdl_course_categories AS cc ON cc.id = c.category" . $where . " LIMIT ?, ?";
$result = $mysqli->prepare($query);
call_user_func_array(array( $result,"bind_param") , array_merge(array(
$valuesPrep ) , $values));  }}
$result->execute();
/* bind result variables */
$result->bind_result($Category, $Course, $Name, $EmployeeID, $Activity, $State);
/* fetch values */
while ($result->fetch())
{
$orders[] = array(
'Category' => $Category,
'Course' => $Course,
'Name' => $Name,
'EmployeeID' => $EmployeeID,
'Activity' => $Activity,
'State' => $State
);
}
$result = $mysqli->prepare("SELECT FOUND_ROWS()");
$result->execute();
$result->bind_result($total_rows);
$result->fetch();
$data[] = array('TotalRows' => $total_rows,'Rows' => $orders);
echo json_encode($data);
/* close statement */
$result->close();
/* close connection */
$mysqli->close();
?>

index.php

<script type="text/javascript">
$(document).ready(function () {
// prepare the data

var theme = 'metro';

var source =
{    datatype: "json",
datafields: [
{ name: 'Category', type: 'string'},
{ name: 'Course', type: 'string'},
{ name: 'Name', type: 'string'},
{ name: 'EmployeeID', type: 'string'},
{ name: 'Activity', type: 'string'},
{ name: 'State', type: 'string'}
],
url: 'data.php',
cache: false,
filter: function()
{
// update the grid and send a request to the server.
$("#jqxgrid").jqxGrid('updatebounddata', 'filter');
},
root: 'Rows',
beforeprocessing: function(data)
{
source.totalrecords = data[0].TotalRows;
}
};
var dataadapter = new $.jqx.dataAdapter(source, {
loadError: function(xhr, status, error)
{
alert(error);
}
}
);

// initialize jqxGrid
$("#jqxgrid").jqxGrid(
{
source: dataadapter,
theme: theme,
width: '90%',
pagesize: 100,
filterable: true,
autoheight: true,
pageable: true,
virtualmode: true,
rendergridrows: function()
{
return dataadapter.records;
},
columns: [
{ text: 'Category', datafield: 'Category', width: 100 },
{ text: 'Course', datafield: 'Course', width: 200 },
{ text: 'Name', datafield: 'Name', width: 200 },
{ text: 'Employee ID', datafield: 'EmployeeID', width: 100 },
{ text: 'Activity', datafield: 'Activity', width: 100 },
{ text: 'State', datafield: 'State', width: 80 }
]
});
$("#csvExport").jqxButton();
$("#csvExport").click(function () {
$("#jqxgrid").jqxGrid('exportdata', 'csv', 'jqxGrid');
});

});

</script>
</head>
<body class='default'>
<div id='jqxWidget'">
<div id="jqxgrid"></div>
<div style='margin-top: 20px;'>

<br />
</div>
<div style='margin-left: 10px; float: left;'>
<input type="button" value="Export to CSV" id='csvExport' />
</div>
</div>

Любая ссылка или помощь будет высоко ценится.