Re: Ugly error on main page
Try creating a new role with this plugin (v 1.0 compatible)
https://bbpress.org/plugins/topic/role-manager/
Then use this plugin to set it up so that your new role always has access to all forums:
https://bbpress.org/plugins/topic/hidden-forums/
That way, you don’t have to disable registration… new users won’t be able to penetrate the hidden forums, no matter what.