BuddyPress.org

I recommend reading https://codex.buddypress.org/developer-discussions/buddypress-forum/ if you haven’t already…

Peterkirn, did you choose a new installation or an existing one when you updated?

Here’s what’s happening, and how to fix it.

Since you picked new installation, your existing groups forums won’t work, because the forum_id’s they want to post to don’t exist in your new installation.

Basically, New installation = New database tables

If your old forums don’t have any important or relevent topics, or any at all, go back and forth between your “wp_bp_groups” table and your “wp_bp_groups_groupmeta” table, and remove the groupmeta entries with the group_id’s of your existing groups, that have any forum_id.

When you disable and enable the forums, it isn’t creating a new forum for them, it’s just turning off the ability. If you created a forum for an old group, and then create a new installation, the new database tables will be empty and your BuddyPress forums will look at those tables for data. If you use an existing installation, then BuddyPress will continue to use the old bbPress tables and allow an external bbPress installation to access them on its own, while letting the bbPress included with BuddyPress use it for its own purposes.