I'm having the same issue.
OS: Windows 7. (me too)
I tried Firefox 20.0.1, switched today to Firefox 21.0
(me too -- and it happened in each)
I go to takeonestepatatime.proboards.com
and then click the login button and then log in, but it says I'm not logged in.
Have to click the log-in button a second time, but then it tells me I'm logged in before I actually type anything.
This is also happening for me in I.E.8.
HOWEVER, in Opera 12.15, the glitch is NOT happening -- i don't have to click a second time.
I log in once and I'm logged in. And I stay logged in.
In other words, in Opera, the log-in process is happening the way i would expect it to happen.
I just tried Chrome version 26.0.1410.64 and the situation was much different
and way more complicated.
I have a personal policy of always refusing all cookies
-- except for specific sites I want to log in to.
Normally I allow cookies for "takeonestepatatime.proboards.com"
and "proboards.com" and also a completely unrelated proboards forum of which i am a member.
And that is enough.
But Chrome wasn't letting me log in to SOS today at all.
Finally I noticed that Chrome is now sending me to a screen "login.proboards.com" and I kept getting an error message from Chrome(error message was from Chrome and not from proboards)
the message said that i had "too many redirects".
Finally i saw that for Chrome, I had to allow cookies on "login.proboards.com", and, when i did that, it let me in to the Global proboards area.
But I had to log in to the global area with my full email address and it sent me to the proboards GLOBAL "entry room".
And then, once I was logged into my global account, I was able to select SOS and be logged in here.
And once logged in here, in Chrome, i STAY logged in, and don't have to click the log-in button again.
To sum up: For some reason, Chrome wouldn't just let me log in directly to SOS. I had to log in via the global screen and then select SOS. But once I was finally logged in here at SOS, I didn't have to do the "second click" like I did in Firefox and IE. And I had to allow cookies on "login.proboards.com" which I didn't previously need to do.