Ajax上传器错误500

I have a Ajax uploader on my page. It worked fine since yesterday. Now I get an Error 500. In console it says

Failed to load resource: the server responded with a status of 500 (Internal Server Error)

and refers to my upload script:

  <?php
// A list of permitted file extensions
$allowed = array('html','htm');

function save_table_to_json ( $in_file, $out_file ) {
    $html = file_get_contents( $in_file );
    file_put_contents( $out_file, convert_table_to_json( $html ) );
}

function convert_table_to_json ( $html ) {
$document = new DOMDocument();
$document->loadHTML( $html );

$xpath = new DomXPath($document);
$tables = $xpath->query("//*[contains(@class, 'mon_list')]");
$tableDom = new DomDocument();
$tableDom->appendChild($tableDom->importNode($tables->item(0), true));

$obj = [];
$jsonObj = [];
$th = $tableDom->getElementsByTagName('th');
$td = $tableDom->getElementsByTagName('td');
$thNum = $th->length;
$arrLength = $td->length;
$rowIx = 0;

for ( $i = 0 ; $i < $arrLength ; $i++){
    $head = $th->item( $i%$thNum )->textContent;
    $content = $td->item( $i )->textContent;
    $obj[ $head ] = $content;
    if( ($i+1) % $thNum === 0){ 
        $jsonObj[++$rowIx] = $obj;
        $obj = [];
    }
}

    return json_encode([ "Values" => $jsonObj ]);
}


if(isset($_FILES['upl']) && $_FILES['upl']['error'] == 0){

    $extension = pathinfo($_FILES['upl']['name'], PATHINFO_EXTENSION);

    if(!in_array(strtolower($extension), $allowed)){
        echo '{"status":"error1"}';
        exit;
    }

    if(move_uploaded_file($_FILES['upl']['tmp_name'], $_FILES['upl']['name'])){
        save_table_to_json( 'heute_S.htm', 'heute_S.json' );
        save_table_to_json( 'heute_L.htm', 'heute_L.json' );
        save_table_to_json( 'morgen_S.htm', 'morgen_S.json' );
        save_table_to_json( 'morgen_L.htm', 'morgen_L.json' );
        echo '{"status":"success1"}';
    }else{
    echo '{"status":"error"}';
    exit;
}
}

I didn´t change anything on server. In server legit only says there was a error 500.

But how can I fix that ?

Edit:

In Log it says:

[Wed Dec 02 17:31:26.625826 2015] [fcgid:warn] [pid 11680] [client IP] mod_fcgid: stderr: PHP Warning: DOMDocument::loadHTML(): Empty string supplied as input in /upload.php on line 12, referer: /

The second:

[Wed Dec 02 17:31:26.625830 2015] [fcgid:warn] [pid 11680] [client IP] mod_fcgid: stderr: PHP Catchable fatal error: Argument 1 passed to DOMDocument::importNode() must be an instance of DOMNode, null given, called in /upload.php on line 7 and defined in /upload.php on line 17, referer: /

Check your webserver error logs. Depending on webserver, should be in either /var/log/apache or /var/log/nginx.

Those will give you information about what's actually going wrong, and you should be able to resolve it easily enough from there.