I've just had several delightful moments of "oh crap I can't use my account anymore." I even telecommuted to work to use my office PC where I'm still logged in to SO to write this question, but then I figured it by accident.
So I had to clear my browser cache.
Then I tried to log in with my usual LiveJournal OpenID (which I do by clicking the pencil icon and typing just my LJ user name in the small box), but Stack Overflow would keep saying in a red box:
No OpenID endpoint found
I tried several times, one time I instead got that in a similar red box:
but otherwise it was consistently the former.
Turned out, when I enter my LJ user ID into the small box, the big box is auto-populated with
https://<correct path to LJ OpenID>, and that link fails. To fix it, I had to go 'Back' and manually change
http://, only then it logged me in.
Please fix the auto-population to use
http:// in the first place.