BuddyPress.org

Found it. As per the discussion here:

https://core.trac.wordpress.org/ticket/17239

dots in usernames are not allowed in WordPress 3.2.(1). I have changed that now in my install by modifying the function wpmu_validate_user_signup in /wp-includes/ms-functions.php in line 524 from

preg_match( ‘/[a-z0-9]+/’, $user_name, $maybe );

to

preg_match( ‘/[a-z0-9.]+/’, $user_name, $maybe );

I don’t like having to modify a core file, but I like even less that in ticket #17239 it was just decided to disallow dots in usernames. For network sites, which often are associated with username databases, which in turn are often associated with firstname.surname server accounts and email addresses, the dot is an importanrt feature!