Your site seems to no longer have the search widget active, right?
But from your feedback above and my experience with some users I assume it's a theme-related issue, mostly with the file "search.php" from the theme which handles the search results display for all results -- and will then also applied to forum search results. If the code in this file is not after WP coding standards you may end up with a query that cannot display links.
With that said, you may also have problems with search results display for other post types, beside the ones from bbPress 2.x
Could you try to remove or rename temporarily a file like "search.php" or "searchform.php" and see if that helps? Just to clarify if it depends on that file in your theme.
Plugin-wise I cannot do anything about different themes and their handling of search results display. We as developers have built in template handling so it falls back to WP regular results handling - but it can have still issues with post types here!!