Our team just upgraded to TFS 2013 from TFS 2010. After upgraded, I had no permission to edit a new build definition successfully. The interesting part was that when I received the TFS notification email alert about build status, the build controller showed "Unavailable". I could see "Unavailable" default controller as well. It is all related to security setting.
After looking into this issue, I am not a member of Project Collection Build Administrators. After my permission is added, the above issues are going away.