I found something that can help to fix this bug. In the following I use wordpress_ as prefix for my WP tables.
My wordpress_usermeta table has 65 rows, and 65 users are displayed now in the bbPress dashboard. It seems to be a wrong SQL statement for querying the number of users.
Instead of the (by me assumed) select count(*) from wordpress_usermeta there should be used either select count(*) from wordpress_users or select count(distinct user_id) from wordpress_usermeta to get the number of users.
(Oh yes, I know that there will be a where-clause…)
I suspect line 2046 in bb-includes/functions.php, but I’m not familar with the source.