No, theres no problem with any Stack Exchange certificates. The problem is Clamid's. You can recreate the issue by hitting their login site directly as well.

Im no SSL expert, but it doesnt look like they have a wildcard SSL certificate, namely it looks like their certificate is only good for claimid.com and not for openid.claimid.com. This appears to be a recent change, as they moved from claimid.com to openid.claimid.com before and now it seems they are reverting (or simply messed up their certificate setup).
According to the certificate dates, it looks like they requested a new certificate (or a renewal) last Friday (11/16/2011). That is, in all likelihood, when the issues began.
https://
to access the site? That's not supported.