I've been having this problem for nearly a month now; to sign in to my Apple ID and I have not found any solutions.
I have tried all potential solutions from what I have fount on the web. Even a back up from TC did not resolved the problem. With all my other devices (iPhones, iPads, iMacs) I don't have any problem signing in with my Apple ID; only with this one iMac!
Perhaps someone has a good idea to solve this problem of being unable to sign in to this one iMac?