Redundant closed DIV tag in $content variable breaks layout

Hi,
I'm running D6.16 ang getting mad with this issue. Maybe someone have an idea to help me to track down the cause.
I was experiencing broken layout on some page, and I found out that if this is the piece of code in node.tpl.php :

<div class="content">
<?php print $content; ?>
</div>

In some pages what I get in the rendered HTML is a redundant closed div before the content, like:

<div class="content">
</div>
Lorem ipsum dolor....
</div>

I am having the problem only on nodes of some content types. By example not in the default 'page' coming with drupal. But if I create a new content type, identical to 'page', the issue comes out.

The issue is not theme dependant... so not my Zen subtheme. I tried with Garland and Stark. It's there.
I checked in the database node table and that div is not in the db.

Any clue?

Thanks,
GP