BuddyPress.org

You need to upgrade to WPMU 2.9.1.1 to use BP 1.2.

Also since you were using 1.0.3, I would have recommended upgrading to 1.1.3 first before upgrading to 1.2.

One other thing, you might also need the BP Backwards Compatibility plugin:

https://wordpress.org/extend/plugins/buddypress-backwards-compatibility/