"unknown error" creating a Messages account using AIM
I had a perfectly good AIM account and password yet whenever I tried creating an account in Messages using AOL type account, I got the error "An unknown error has occurred". It also happened when I tried to add the AIM account in SystemPreferences->Internet Accounts.
SOLUTION: In my case the email address for the AIM account was a yahoo.com email address. The same email address was used for the Yahoo Internet Account in system preferences. Creating another AIM account using a gmail.com address solved the problem. YMMV
MacBook Pro, OS X Mountain Lion (10.8.5)