The URL is fetched in the context of a guest and we only try to get the title if we get a successful response (200). A guest trying to view a forum they have no permission to would give a forbidden response (403) hence at that point it gives up and can't get a title.
In this particular case, the web server is returning a response which we don't consider to be successful.
It's returning a "303 See Other " response.
Strictly speaking, we could consider 3xx responses as successful, or do further checks to ascertain that is the case, but generally a 3xx response isn't "expected" therefore backing out at this point is probably fair.