I have a site running Drupal 6.20. I noticed that the search form is not working, so after a little inspection I found 2 problems:
- when search_simply($text) is called, the following line
$text = preg_replace('/['. PREG_CLASS_SEARCH_EXCLUDE .']+/u', ' ', $text);
sets $text to NULL (no idea why - can comment it out thou);
- search_index, search_node_links and search_total are empty, and running the cron doesn't fill them up.