[BuddyPress] Member profile page not found

Hi all!

I need help for a little irritating question about buddypress.
I've a Mulsite installation with 8 sites, on a subfolder install, with
-WP 3.4.2
-BuddyPress 1.6.1
- Membershp plugin by MpMuDev

Now i'm getting a really strange issue on this, because I can register to the site, but once i'm registered I can't access to my profile page: i get a 404 page not found error.

Quite frustrating.

Here is my .htaccess file, i think have to fix it, because it's like an apache problem or similar.

RewriteEngine On
RewriteBase /bitmat/
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

I change the permalink structure from default to a custom structure but didn't fixed the problem.

How can i fix it ???

Thanks all!

Graz