Greetings!
Below is the fix for Snow Leopard (10.6.8). It looks like it's going to work with later versions of OS X as well.
It took me awhile to figure this one out. The problem has to do with the lack of "com.apple.facetime: registration" entry in the "login" Keychain of a FaceTime-troubled account, which is likely the administrator account. Standard accounts work fine.
1) Go to System Preferences -> Accounts (unlock it in the lower left corner, if needed) and create a new administrator account on the Mac in question.
--- It is imperative to have at least one administrator account on OS X! --
2) Click on "Login Options" set "Automatic login" to Off.
3) Log out of all accounts (all users) and log in into a newly created (administrator-privileged) account.
4) Go to System Preferences -> Accounts, select the FaceTime-troubled account/user, the one in which "registering device does not have appropriate credentials" and remove a check mark next to "Allow user to administer this computer".
5) Press OK to confirm that "changes will take effect after a computer restart". So - reboot your mac.
6) Now - log in into the FaceTime-troubled account/user, launch FaceTime, sign in with your apple id and make a call. It has to ring a few times... no need to actually speak with somebody. The "registration" item is now created in the Keychain "login". Quit out of FaceTime.
7) Reboot - so "login" Keychain retains the registration for FaceTime (imagent specifically).
8) Go to System Preferences -> Accounts and put the check mark back to "Allow user to administer this computer" in the account/user you have removed it from (FaceTime-troubled account/user).
9) Do step 5 (so the change takes effect).
10) Go to System Preferences -> Accounts and delete the account you created in step 1. Restore Automatic login as well, if needed.
I hope this helps.
All the best,
A:>
