There is nothing that you can do to fix it, the login servers are down/crashed/overloaded. Anything that 'works' is just coincidence, this is a login server issue, there is nothing that any player can do. Flushing DNS cache, changing server, etc, are just coincidental. It will let you in sometimes, and not other times, even if you do none of these things, you will still have a small chance of 'getting in' (At least up until lately, when it seems that the login server has just crashed completely now, at least from what I see)