BuddyPress.org

== I have installed and reinstalled WP3.1.2 + BP 1.2.8 from scratch ==

When you say you did it from scratch, do you mean you uploaded WP files manually and not used webhost script like fantastico and the like? If you used the webhost script, I suggest you drop tables from DB, delete .htaccess file and re-install WordPress manually via FTP/cpanel. I’ve seen dozens of BP install problems arising from the fact that WP was installed using those webhost scripts.