My account works pretty well on the other three sites.
But when I log on to Super User, I get:
This is an OpenID we haven't seen before: [openid]
What gives?
Don't worry about it. Go on and create a new account. Then, in the accounts tab of your profile, associate your SU account with one of your other accounts and you can copy your profile information over.
We improved the message so it now says
This is an OpenID we haven't seen on {website name} before:
But Kyle's answer, above is 100% correct.
As of June 2018, openid is being depreciated on the network so its unlikely that a user will see this error message.