Relevant help center pages:
Why do I see the message “OpenId endpoint not found” when I try to log in?
If you cannot log into your OpenID provider, or if you know your provider has ceased supporting an OpenID, please create a new account and then contact us to request an account merge.
Still, it's better to prevent problems if you know this will happen, so…...
You should also be able to log in with any other OpenID provider that you have registered the same email address with, and you will automatically be logged in to your account if you have a verified email address.