BuddyPress.org

I had a similar issue a week ago and was instructed to go the group admin page and click on Admin > Group Settings and then…

1. Make sure you have the “Enable Discussion Forum” check box checked. (I’m sure you do but just in case).
2. Now, uncheck the check box and click “Save Changes”
3. Check the checkbox again and click “Save Changes” again.

I know it sounds crazy but this completely fixed my forums and people could start posting topics again immediately.