I reinstalled default 2020 WP theme, deactivated all plugins (double checked) but bbpress and buddypress, although I’ve kept wpmu admin dashboard plugin… I am on a very fresh install (less than 24 hours)
Buddypress seems to work ok, activity, group and email tabs under admin menu of the admin network page, main site shows forum tabs only and subsite 1 (?id=2) holds buddypress frontend pages.
Soooo, maybe a clue here : Why does the group edit screen say “aucun forum” (no forum) attached to group when in the meantime I can see the forum tab and the input text boxes of the first subject when group is not hidden?
Many infos here, sorry for that, but I guess I can’t be more accurate than that in my perception of the problem.
Hope this helps you to understand the problem better.