If you choose to upgrade the hard drive on your system either for a faster drive or for one with more space, you may find Apple's new FaceTime videoconferencing application will no longer authenticate. When launched, the program will give an error stating "The server encountered an error processing registration. Please try again later."
This has been shown to happen when people have upgraded from magnetic hard drives to SSD drives, but may also happen with other hardware changes as well. The problem is because Apple uses a certificate system for authenticating your system with FaceTime, and the certificates are generated and approved for a specific hardware configuration so only approved systems will be able to use your account.
If this happens to you, even if you have not performed any hardware changes, the easiest way to fix the problem is to remove the certificate that FaceTime is using, and have the service generate a new one for your system. To do this, follow these steps:
Open the Keychain Access utility
This utility is located in the /Applications/Utilities/ folder.
Locate the certificate issued by "Apple iPhone Device CA"
This certificate will likely be named as a UUID (universally unique ID number), which will be a string of characters and numbers separated by dashes. If you select the certificate you will see information on it at the top of the Keychain Access window, which should show it being issued by "Apple iPhone Devices CA."
The certificate can also be located by comparing its expiration date (October 20, 2011, in the example here) to the date modified for the FaceTime application itself (seen by getting information on the application), which should be a year earlier than this date.
Delete the certificate
Once this is done, close Keychain Access and relaunch FaceTime. Supply your credentials to use the service and the service should now log you in.