[Sarah Gooding]

This fix worked for me:
Go to: phpmyadmin > yourdatabase > wp_bp_groups_groupmeta

Find the group you’re attempting to post to and then delete the forum_id value. Go back to your group on the front end. When logged in as admin of the group, go to settings and re-enable your forums. It will save a new forum_id for you and your posts should start working. This is a solution for BuddyPress groups that are starting a fresh new forum, not for groups with forums that have existing posts.

I posted about it here: http://wpmu.org/daily-tip-fix-for-buddypress-group-forums-error-when-creating-a-new-topic/