I set the logout redirect to forum/latest there is no error. Error comes back, if you are going to visit the forum without being logged in. After thinking about it, I changed the default view back to latest. Otherwise no guests could read the forum.
I still have no real insight (working on it) - but maybe it is possible to add a warning text at the admin panel not to put unread first. Or maybe its is even possible to do an redirect from unread to latest or the second configured view, if you are not logged in? It’s not urgent at all since there is a workaround for it.