I've fixed it myself by adding the following code in the com_content/html/article/default.php:
<?php
if (!empty($this->item->pagination) && $this->item->pagination && $this->item->paginationposition && $this->item->paginationrelative) :
echo $this->item->pagination;
?>
<?php endif; ?>
Before this php code that was already in there:
<?php // Content is generated by content plugin event "onContentAfterDisplay" ?>
<?php echo $this->item->event->afterDisplayContent; ?>
The next problem is that the chevrons are not showing up in the previous/next buttons (when I diasble the CSS font-family: 'icomoon' !important; on that button it shows up in some browsers, but not every time)