BuddyPress.org

First of all, since you are installing a premium, 3rd-party DB sharding tool, you need to contact the people at WPMU DEV Premium and ask them for support. It seems like all was fine with your standard WPMU + BP install until you switched to using their multi-db plugin.

Secondly, in case you are not aware, there is a free sharding tool–HyperDB. In fact, this site (BP.org) is using it. Also, there is a great two-part article on how to use HyperDB written by Trent Adams, one of the moderators on this and other WP product forums.