Members are trying to access the forum via the 'Forum Base URL'. But once they go to that page, the forum is not displayed.
This error can only be replicated using a 'Member' account.
When you install wpForo it comes with correct settings and configurations, so all users can see the forum page calling yourdomain.com/community/ URL (base URL). I think you have done some incorrect permission configuration or something else. I cannot follow it and I have no chance to see why it works in that way. You should provide more information.