After bbpress version 2.1.3 there was a change to the default user role, which is now Participant. After the upgrade the user Forum Role will be listed on some installations as “No Role for These Forums,” which causes those users to see a 404 error page when visiting Private or Hidden Forums. Even the Administrator user is impacted.
On the bbpress Settings page there is an option called Auto role, that says “Automatically assign default role to new, registered users upon visiting the site.” Check the Auto role option, and users will automatically have their role changed from “No Role for These Forums” to Participant when they visit the site, and login. The Administrator should have the role of Keymaster again. Once the role is updated automatically the visitor can view the forums again.