Out of curiosity, have you checked the top left of the page to make sure you were using the North America region setting? Image below:
http://puu.sh/dbe5l/3520f0f98c.png
I get a similar issue to OPs, if not the same. At random times, I come here and I'm not logged in, and when I try to, it keeps constantly telling me the CAPTCHA was wrong or my password was wrong after I've made sure I have them correct multiple times, until it finally just says "Internal server error" repeatedly. And then at random times, I come back here and it already has me logged in.
It's like the boards literally decide when I'm allowed to be logged in and when I'm not. I never realized it might be connected to visiting old forums though.
And I'm pretty sure my region setting hasn't changed.