Error on comments_template

If anyone could give me a help it would be very nice. 🙂

On my blog (http://portaviii.8esfera.com) I’ve a little problem. When I open a page (http://portaviii.8esfera.com/?page_id=11) I obtain a error on the bottom of the page:
Warning: comments_template(/home/oitava/public_html/portaviii/wp-content/themes/default): failed to open stream: Success in /home/oitava/public_html/portaviii/wp-includes/comment-functions.php on line 24

Fatal error: comments_template(): Failed opening required ‘/home/oitava/public_html/portaviii/wp-content/themes/default’ (include_path=’.:/usr/lib/php:/usr/local/lib/php’) in /home/oitava/public_html/portaviii/wp-includes/comment-functions.php on line 24

How can I solve this?
Thanks!
Paulo Brito