I'm getting this when I try to sign up for academia.se:
We updated DotNetOpenAuth (the library we use for OpenId authentication) to version 4 recently. Something in that update broke OpenId 1 delegation (which Wordpress uses); OpenId 2 delegation continued to work (as Shog9 notes in a comment on your question).
We'll have to figure out whether this is a problem in DNOA or if we just screwed up something; for the time being, I have reverted the update. It should be back to working everywhere now.