I've spent uncountable hours trying to get these apps to log into my iCloud account so that I could use them. Everything I tried failed, not that it was my failure, but Apple's for making it so that it got so screwed up.
I checked the Apple support forums, and I found dozens of posts from people with exactly the same problems. Others would advise them to try basically everything that I'd tried, and sometimes bizarre and unnecessary things, but the members with the problems would post that those fixes did not work. Often people would say "this worked for me," but those who tried it would come back and say "but not for me." Everyone was getting really frustrated.
The one thing I had not done was log out from iCloud. Why? Because as soon as you click "log out," you start getting countless dialogs saying things like "this will delete all iCloud data from your Mac if you choose to continue." You click "OK," and then "are you sure you want to erase all your iCalendar files? Are you sure you want to delete your Notepad files? ...Contacts?" Previously, when I started getting those scare messages, I would cancel the whole thing. But that's (partly) why I never got it fixed. You have to log out of iCloud, set aside your old keychain file, and start over.
Then I went to FaceTime, which was pretty easy to fix at that point, and Messages.app, and got them working. Took a few tries at Messages, but soon I was sending texts.
Later, I put back my old Keychain file and logged back into iCloud, and after several tries at it, I was able to get them all working again... together. At this point Messages took a LOT more work. Several restarts, hundreds of attempts to log in, finally I started over enough times that something "took," and it all worked. I really didn't think it was going to, for a while. But as of now, I can safely say that I'm sending messages, doing FaceTime, and using my old Keychain entries, as well as using all my old iCloud data, and it's all working as it should have when I installed Yosemite and El Capitan.
To recap the steps I took, in case anyone else is having this problem:
- • System Preferences / iCloud: Sign out of iCloud
• Rename: ~/Library/Keychains to ~/Library/Keychains.old
• Shut Down the Mac
• Reboot the Mac (It didn't work until I actually shut it down, as opposed to restart, so save yourself a reboot cycle and shut down first)
• FaceTime.app: Set up preferences and numbers.
• iMessages / Preferences. (Login and manage accounts, etc. until you've got it all working)
• System Preferences / iCloud (Login)
• ~/Library/Keychains: swap the names so that your old keychain is now the current one, and the recently created keychain is now "Keychains.old"
• Notice that you will need to have your iPhone handyd8sf
The key is to understand that logging out of iCloud does not eliminate the things it deletes from your Mac. It will give you the option of saving your Safari keys, such as credit cards, site passwords, etc. Some things it saves locally, while all your calendar, contacts, and notes are still saved remotely on iCloud, itself, for the next time you log back in.
Just move slowly and think through everything. A false move MAY cause you data loss. Also, you'd better have your passwords handy. You'll be using them!
Shooshie