BuddyPress.org

@chrchcol – this seems like the same issue – http://premium.wpmudev.org/forums/topic/fatal-error-when-trying-to-access-group-forums-in-buddypress

From Jeff in that thread:

Thanks so much for all of your input… turns out all that I needed to do was re-do the setup for the forums pointing it to the old bb-config.php file.

After completing that step, all the forums worked again without a hitch and no data was lost.