BuddyPress.org

Login to the WP backend, navigate to “BuddyPress > Component setup”. Disable bbPress forums. Save and re-enable bbPress forums.

Then navigate to the group in question, navigate to the group admin area, click on “Group Settings”. Uncheck “Enable discussion forum”, save and then re-check “Enable discussion forum” and save.

Now try to post a forum topic.

See if that works.