BuddyPress.org

Try resaving your permalinks as something other than default.

And/or check your .htaccess file in the website root directory (where wp-config.php is) for a block like the following:
`
# BEGIN WordPress

RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

# END WordPress
`