It seems the problem is very inconsistent. In all honesty I have been having it on and off less frequently for the past month or so, but figured it was because of the big upgrade you had mentioned. I believe I experienced similar when Eugene Goostman hit the news and overloaded the forum with visits. The problem did not become so extreme again until, come to think of it, in the week the results of the Loebner Prize were anticipated. Might this have the same cause? Different to then, the forum is not slow.
For some miraculous reason today I can, sometimes, access the forum and main page without a hitch. I don’t seem to have the problem on my Windows Vista Internet Explorer computer that I rarely use, at least not in recent memory. I do still have it, on and off, on my Windows 8 Firefox computer.
Let us ignore the doubling of the 404 and my 404’ed post, as they both only occurred once, and focus on the greater symptoms.
There isn’t a specific URL from which I am visiting the site. I typically just open Firefox, with Deviantart.com as start page, and type the address “chatbots.org” or “chatbots.org/ai_zone” directly into the address bar. Or when I Google “chatbots forum” and click on the link listed there, I also get the 404. If I instead access e.g. “chatbots.org/viewforum/9” by typing it in my address bar, there is no problem. However, from any page on the forum, if I clicked either the “Forums” tab, the “Directory” tab or the “Forum home” link at the top of a thread, I got a 404. Even the manual entry of “chatbots.org/viewforum/9” then provides a 404 from then on, until I close the browser.
I manually deleted my cookies, but this has no effect.
Hang on, when I type “https://www.chatbots.org” as the url, it accesses well. When I type it as “http://” without “-s” or as “www.etc”, I get the 404. Google also, links to “www.chatbots.org”, not “httpS://www.chatbots.org”. My Firefox browser may be using history: Have you changed from http to https recently?