Happened to me last week, but seems fine now. You could always save your password with your web browser like Paul suggested, then it isn't tied to any problems with the DIS. I use firefox and it will offer to save your password for you. I am also have the issue with threads not correctly remembering where I last read on that particular thread.
That happens on my work computer...all of a sudden it won't remember me and I always get that "If this is your first time..." message. My home computer seems fine. I don't post that much though, so not a big deal for a lurker
I did try this at home and it was still playing up.
I signed in, replied to a PM and I was logged out when I tried to send it - luckily I went back and remembered what I'd written.
Kind of annoying but I just lurked and didn't post