After a jenkins core + plugins upgrade a few weeks ago, OpenId started failing with
"Unable to find an on-going OpenID session. Could it be that you have multiple host names for your Jenkins and you started the authentication in one host name and landed back on another? If so configure the correct Jenkins root URL so that those two host names will be the same"
I can log properly when using a different browser so I doubt the problem is configuration only.
On my main browser, I use 2 different gmail accounts simultaneously, on 2 different tabs (perso & company). Jenkins/openid is attached to the company one. Maybe the issue is caused by that. Or by cookies. I will try to clear things up a bit, downgrade the plugin etc.