Fatal error: Call to undefined function

Hi!

I have just installed a WordPress blog in a new web server. The index seemes to work fine, the pages too, but I get one error. When I go to a permalink to a single entry (single.php), I just get this message:

Fatal error: Call to undefined function wp_get_pages() in [URL]/header.php on line 35

I have Googled this for ever now, and just can’t get what’s wrong. I have installed WordPress many times before. I have also tried to run the theme on an other server, and it works just fine there. When I use a standard theme at this server, on the other hand, the problem is gone.

Any ideas?

Here is my single.php:

<?php get_header(); ?>

<div id="content">

 <?php if (have_posts()) : while (have_posts()) : the_post(); ?>

  <div class="innlegg" id="post-<?php the_ID(); ?>"> 

 	<h1><?php the_title(); ?></h1>

 	 <div class="postInfo">
 	 	 Publisert: <span class="dato"><?php the_time('j. F, Y'); ?></span></div>	 

 	 <?php the_content(''); ?>

	<div class="tags"><?php the_tags( '' ); ?> </div>

  </div><!-- avslutt innlegg -->

    <?php comments_template(); ?>

  <?php endwhile; else: ?>

  <?php endif; ?>

</div><!-- end #content -->

<?php get_sidebar(); ?>

<?php get_footer(); ?>

Thanks.