I had the same issue (failed login to Area 51) with a non-SE OpenID account. FWIW my OpenID URL uses a delegate. I could still log in everywhere else on SE.
I switched from Firefox to Chrome and the login worked.
My installation of Chrome is barely configured, has no plugins and I rarely use it, so it also has few security restrictions (e.g. third party cookies), few cookies hanging around, etc. So, this doesn't really diagnose the problem, just suggests that disabling plugins, flushing your current browser and/or using an alternate browser should help.