WCF::handleException() Unhandled exception: Class 'DOMDocument' not found

#0 /var/www/ipod-forum.de/html/lexicon/lib/action/EntryPdfExportAction.class.php(82): Dompdf\Dompdf->loadHtml()
#1 /var/www/ipod-forum.de/html/wcf/lib/action/AbstractAction.class.php(49): lexicon\action\EntryPdfExportAction->execute()
#2 /var/www/ipod-forum.de/html/wcf/lib/system/request/Request.class.php(58): wcf\action\AbstractAction->__run()
#3 /var/www/ipod-forum.de/html/wcf/lib/system/request/RequestHandler.class.php(139): wcf\system\request\Request->execute()
#4 /var/www/ipod-forum.de/html/lexicon/index.php(11): wcf\system\request\RequestHandler->handle()
#5 {main}