Here's one solution to the irritating OS X Server "You are unable to log in to the user account 'xxxx' at this time. Logging in to the account failed because an error occurred" error.
Check the client machine. The Mac that wouldn't log on had somehow flipped its network settings to "DHCP" from the static IP setting it had been on. Checking what the IP and DNS addresses should be on the server side and then plugging them into the client worked like a charm!
