OK I've found the cause, my Wordpress loops.
I use a loop in the index.php to display my posts and another in the sidebar.php in order to show the last post of one category.
Here is the sidebar.php loop which is called before the index.php:
<?php $my_query = new WP_Query('category_name=pensees&showposts=1'); while ($my_query->have_posts()) : $my_query->the_post(); $do_not_duplicate = $post->ID;?>
<div id="post-<?php the_ID(); ?>">
<h3 class="sidetitle"><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></h3>
<div class="penseecontent">
<?php the_content(__('(more...)')); ?>
</div>
</div>
<?php endwhile; ?>
Here is the index.php loop:
<?php if (have_posts()) : while (have_posts()) : the_post(); if( $post->ID == $do_not_duplicate ) continue; update_post_caches($posts); ?>
<?php if (in_category('7') && is_home() ) continue; ?>
<div class="post" id="post-<?php the_ID(); ?>"><div class="post-other"><div class="post-ie">
<h3 class="storytitle"><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></h3>
<div class="storycontent">
<?php the_content(__('(more...)')); ?>
</div>
<?php comments_template(); // Get wp-comments.php template ?>
</div></div></div>
<?php endwhile; else: ?>
<?php _e('Sorry, no posts matched your criteria.'); ?>
<?php endif; ?>
If I call the sidebar.php after the plugin works fine. But my design is messed up.
By the way this causes this other problem too.