500 Error for Posts and Post Comments

I’ve been working on my blog for a few weeks now, and I’ve just realised that blog single posts and comment posts are returning a 500 error in my browser. Permalinks are turned on with the value being %postname%/

I have read elsewhere and tried the following solutions I’ve found on this forum and others;
– .htaccess to include mod_rewrite solution
– modified httpd.conf to allow mod-re_write
– wp_posts database changes to allow negative values

Website is http://www.262oh.com.au – (don’t be too critical, it’s still being developed). Any thoughts? I don’t susepct it’s a .htaccess problem, as other pages are working. It seems only to happen to single and comments pages.

Below is the code for single.php:

<?php get_header(); ?>
 <div id="content">
   <?php
	$blog = get_cat_id(get_option('rascals_blog'));
	global $more;
	$more = 0;
    if (have_posts()):
    ?>
    <?php while (have_posts()):
        the_post();?>
    <div class="entry">
      <div class="entry-top"> </div>
      <div class="entry-content">
        <h1><a href="<?php the_permalink();?>" rel="bookmark" title="<?php the_title_attribute(); ?>"><?php the_title();?></a></h1>
       <?php the_content('[ read more ]');?>
      </div>
      <!-- end entry-content -->
      <div class="entry-bottom"> </div>
      <?php if (in_category($blog)) : ?>
        <div class="post-meta-data">
         <p>Posted <?php the_time( 'M j, Y' ); ?> in <?php the_category( ', ' ); ?> </p> <a href="<?php the_permalink() ?>#comments" title="Comments" class="comments"><?php comments_number( '0','1','%' ); ?></a>
      </div>
      <? endif; ?>
    </div>
    <!-- end entry -->
    <?php endwhile;?>
    <?php else :?>
      <div class="entry">
      <div class="entry-top"> </div>
      <div class="entry-content">
        <h1>Error - 404</h1>
         <p>
            Sorry the page you are looking for does not exist
        </p>
      </div>
      <!-- end entry-content -->
      <div class="entry-bottom"> </div>
    </div>
    <!-- end entry -->
    <?php endif;?>
     <!-- comments -->
     <?php if( 'open' == $post->comment_status ) comments_template(); ?>
    <!-- end comments -->
  </div>
  <!-- end content -->
  <div id="sidebar">
		<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('Single Post Sidebar') ) : ?>
		<?php endif; ?>
  </div>
  <!-- end sidebar -->
</div>
<!-- end wrapper -->
<?php if (!in_category($blog)) {include(TEMPLATEPATH . '/footer.php');} else { include(TEMPLATEPATH . '/footer-blog.php');}; ?>
</blockquote>