I can authenticate to Trailhead with the Dev org credentials I use for all my community profiles, but when I try to authenticate that org with the "Connect to Your Developer Edition" button above the Challenges, I am led through the OAuth, but then end up at this URL (brackets and ellipses added to remove codes):

https://dfc-org-production-site.secure.force.com/orgtrailheadauth?code=aPrxl0v[...]e6N2G._regTiYVbjA%3D%3D&state=origin_host%3Ddfc-site.herokuapp.com%26user_id%3D223791%26ts%3D1413906236%[...]efc8c72e8a1858f219e

The message on the page is:

Authorization Required 

You must first log in or register before accessing this page. 
If you have forgotten your password, click Forgot Password to reset it. 

The URL parameters look a little malformed, with URL encoded ampersands and equals instead of the actual characters, and perhaps a missing ampersand and/or parameter name between the end of the "code" parameter and the beginning of the "user_id".