It is possible to use bbPress without BuddyPress. bbPress lets you add forums to your site and the groups component from BuddyPress adds functions for usergroups to have their own forums. You don’t need the groups component active to use the forums though, it just adds group functions.
The problem of not being able to switch the group visibility is known, but there hasn’t been an update to fix it yet.
You can get around the bug with making private/hidden forums by using the Bulk Edit. So on your Forum list in WP-Admin tick more than one forum, choose the Edit option from the dropdown and that will let you set it to private/hidden (you can change one back to public, or delete it completely in the normal way).
thank you for advice but this works only for private forums, right? I´d like to create a hidden forum so that users with “Participant” role have no access.
Yes, you are correct, unfortunately there’s no easy way to do get hidden at the moment. However, I’ve just tried manually reverting the changes from 2.4 that are causing the problem and it looks like that works.
All I’ve done is revert the 3 changes here: https://bbpress.trac.wordpress.org/changeset/5060/trunk/includes/forums/functions.php
I’ve done it fast and dirty by editing core and replacing the new lines with the old ones which you really shouldn’t do if you can avoid it as it will be overwritten next update….but hopefully next udpate will fix the problem anyway. I can’t see it causing problems anywhere else and that code was only changed for best practice I think so should be OK.