BuddyPress.org

YEZZ, it works again. Thanks for your help again!
The only things what are needed:

In the root of BuddyPress (version 1.2.3):
In bp-blogs.php, change rule 379 of
if ( (int)get_blog_option( $blog_id, ‘blog_public’ ) || !bp_core_is_multisite() ) {
IN:
if (get_blog_option( $blog_id, ‘blog_public’ ) > 0 || !bp_core_is_multisite() ) {

and change rule 427 from
if ( (int)get_blog_option( $recorded_comment->blog_id, ‘blog_public’ ) || !bp_core_is_multisite() ) {
IN:
if (get_blog_option( $recorded_comment->blog_id, ‘blog_public’ ) > 0 || !bp_core_is_multisite() ) {

and the MU plugin:
ds_private_blog.php: http://wpmudev.org/project/More-Privacy-Options

With regards Anja(afoka)