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Nice! That was the most stupid mistake ever… I found the answer myself. Believe it or not… I had put this in sidebar.php
<div class="box">
<h3>Categories</h3>
<ul>
<?php wp_list_cats('sort_column=name&hierarchical=1'); ?>
<ul>
</div>
instead of:
<div class="box">
<h3>Categories</h3>
<ul>
<?php wp_list_cats('sort_column=name&hierarchical=1'); ?>
</ul>
</div>
I can’t believe I spent like 3 hours looking on the net and rereading the code over and over to figure that out… ! Duh!