BuddyPress.org

Okay, at this stage I’d say it’s time to clean house.

If this is a test dev install–meaning you do not have it in production mode–then I recommend deleting absolutely everything. This means the database file as well.

Install a clean, new copy of WPMU 2.7.1 plus BuddyPress RC2 and see what happens. After WPMU is properly installed, then install BuddyPress RC2 using the plugin browser as described here: https://codex.buddypress.org/getting-started/installing-buddypress/

Let us know how that goes.