I've embedded it on my website
here and have the same problem - I cannot log in. I know I'm entering the right info as it doesn't complain about them and does if I use other credentials. It takes the the credentials but doesn't actually log me in as it doesn't show I'm in nor can I post.
All of this is with Chrome. But, it does work with Firefox, which is exactly what we've seen before - works with Safari only, but not other browsers.
By the way, when security was apparently turned off on my regular forum a few weeks ago I had the same problem. So it seems like it is something to do with security.