BuddyPress.org

Were you running WPMU 2.7.1-beta1 before upgrading to the public version of 2.7.1?

If so, did you try using the auto update feature? If you did, it will not work since the version is already 2.7.1.

Instead, you have to do it the old fashion way: delete what is not needed, upload new files, then log in as site admin and got to Site Admin > Upgrade and finish the upgrade. Then, install BuddyPress, activate, and do the various theme directory changes.

Also, are you using a brand new wp-config.php file–the one that comes with WPMU 2.7.1? Or, did you use an older version?

If so, there is an additional define line added to WMPU’s config file as of 2.7.1:

define(\'SITE_ID_CURRENT_SITE\', 1);