Note: I figure that the temporary banning for IPs for requesting pages too fast and recurrently is not currently in effect (otherwise my scrapper account would have encountered issues).
I did a quick test, it's working:
Too fast / overloaded (503)
Most likely, you are accessing the forum too quickly. You are only allowed to access at a rate of one pageload per second on average (somewhat higher bursts).
It is also possible that the server is overloaded or temporarily down.