Can you post a few lines near line 689 from viewtopic.php on your site?
//
// Send vars to template
//
$template->assign_vars(array(
'START_REL' => ($start + 1),
'FINISH_REL' => (isset($HTTP_GET_VARS['finish_rel'])? intval($HTTP_GET_VARS['finish_rel']) : ($board_config['posts_per_page'] - $start)),
'FORUM_ID' => $forum_id,
'FORUM_NAME' => $forum_name,
'TOPIC_ID' => $topic_id,
'TOPIC_TITLE' => $topic_title,
'PAGINATION' => $pagination,
'PAGE_NUMBER' => sprintf($lang['Page_of'], ( floor( $start / $pagination_ppp ) + 1 ), ceil( $total_replies / $pagination_ppp )),
'POST_IMG' => $post_img,
'REPLY_IMG' => $reply_img,
'PRINTER_IMG' => $printer_img,
'L_AUTHOR' => $lang['Author'],
'L_MESSAGE' => $lang['Message'],
'L_POSTED' => $lang['Posted'],
'L_POST_SUBJECT' => $lang['Post_subject'],
'L_VIEW_NEXT_TOPIC' => $lang['View_next_topic'],
'L_VIEW_PREVIOUS_TOPIC' => $lang['View_previous_topic'],
'L_POST_NEW_TOPIC' => $post_alt,
'L_POST_REPLY_TOPIC' => $reply_alt,
'L_PRINTER_TOPIC' => $printer_alt,
'L_BACK_TO_TOP' => $lang['Back_to_top'],
'L_DISPLAY_POSTS' => $lang['Display_posts'],
'L_LOCK_TOPIC' => $lang['Lock_topic'],
'L_UNLOCK_TOPIC' => $lang['Unlock_topic'],
'L_MOVE_TOPIC' => $lang['Move_topic'],
'L_SPLIT_TOPIC' => $lang['Split_topic'],
'L_DELETE_TOPIC' => $lang['Delete_topic'],
'L_GOTO_PAGE' => $lang['Goto_page'],
'S_TOPIC_LINK' => POST_TOPIC_URL,
'S_SELECT_POST_DAYS' => $select_post_days,
'S_SELECT_POST_ORDER' => $select_post_order,
'S_POST_DAYS_ACTION' => append_sid("viewtopic.$phpEx?" . POST_TOPIC_URL . '=' . $topic_id . "&start=$start"),
'S_AUTH_LIST' => $s_auth_can,
'S_TOPIC_ADMIN' => $topic_mod,
'S_WATCH_TOPIC' => $s_watching_topic,
'S_WATCH_TOPIC_IMG' => $s_watching_topic_img,
'U_VIEW_TOPIC' => append_sid("viewtopic.$phpEx?" . POST_TOPIC_URL . "=$topic_id&start=$start&postdays=$post_days&postorder=$post_order&highlight=$highlight"),
'U_VIEW_FORUM' => $view_forum_url,
'U_VIEW_OLDER_TOPIC' => $view_prev_topic_url,
'U_VIEW_NEWER_TOPIC' => $view_next_topic_url,
'U_POST_NEW_TOPIC' => $new_topic_url,
'U_PRINTER_TOPIC' => $printer_topic_url,
'U_POST_REPLY_TOPIC' => $reply_topic_url)
);
the only other culprit could be page_header_printer.php; you can look through the template files in the scripts and see if you've the right path/name.
I had kind of ruled the path out because it works if the user is
not logged in. Would that thinking not be correct?