BuddyPress on subfolder in Multiste

I installed BP on a default multisite root installation and did some testing.
After that I deleted all tables of my database and did a clean new install of WP (3.5.1)
I installed Multisite and created 2 subsites.
/store and /community
The latter I want to use for BP. I activated BP in the /community site and it gives me the yellow bar at the top to start the wizard. After clicking that I get this message:
"You do not have sufficient permissions to access this page."

I searched on various forums and other dev sites but there does not seems to be a full solution for this.
Can you give me a hint?
I have this in my config file:
define ('BP_ROOT_BLOG', 3 ); where 3 is the id of my community subsite