Register issue with BuddyPress installed on a sub-site

Hello,
In my WPMU network, to have a better structure, I installed the BuddyPress on a sub-site. To archive that, I followed the guide in buddypress official site, adding the code in wp-conf.php
define ( 'BP_ROOT_BLOG', 21 );
So, my buddypress site is http://buddy.mydomain.com.

The problem is:
When users wants to register, they will be redirected to http://buddy/mydomain.com/register. In the register form, the blog part shows, http://____.mydomain.com will be their site name.
- However, after submission, http://subsitename.buddy.mydomain.com is created!
- And, after activation, the password users have set during registration does not work at all.

What may cause this issue and how to solve this?

Regards
YZ