I have successfully registered my website on https://openid.stackexchange.com.
But when I go to my profile on any Stack Exchange site and try to add that as a new log in, it doesn't work.
On My Logins I click add more logins …, then I add my URL. I get the confirmation page and confirm. After that I'm redirected to my profile. But the new provider isn't listed there.
The log on https://openid.stackexchange.com/user says Authenticated to wordpress.stackexchange.com, so that part works.
When I try to log in with that new provider, a new account is created, and it's not linked to my existing account.
I tested that on Stack Overflow, WordPress SE, and Chess SE. Browsers: Vivaldi, Internet Explorer 11, and a vanilla Firefox installation. No extensions, default cookie settings.
I also searched for related topics here, but none of them describe the exact problem I have.
It looks like the process that stores the OpenID is just broken.