Hi @dirk-k,
We have updated the T4 System Plugin and this issue will be included in the next version.
You can also manually fix it by opening the blog_item.php file in the "\plugins\system\t4\themes\base\html\com_content\category" path and replace the code below:
<?php if ($params->get('show_intro')) : ?>
<div class="item-intro">
<?php echo $this->item->introtext; ?>
</div>
<?php endif; ?>
by
<div class="item-intro">
<?php echo $this->item->introtext; ?>
</div>
Then save and reload your website to view the changes.
Kind regards