Q: El Capitan Update - App store Sign in?

I just wanted to share this solution after spending what seems like weeks trying to solve this issue. I contacted the apple support team (lost count on the number) and eventually solved this by chatting with a colleague for 30 secs. It was that simple.

I read many threads on many forums so I know this is still out there and although there are other causes this worked for me:-)

Problem: could not sign into App store, icloud or itunes from the MAC. although I could sign in online through a browser?

Solution: Go to all files in Finder. Select the Hard drive. Select users Folder. Click once on shared. Then right click, select get info. Change all sharing permissions to Read & Write.

Done.

Hope this helps