The clients are not getting latest address book. when
you try to download the address book file using browser
you get "Service unavailable" issue.
Solution:
The application pool LSGroupExpAppPool might be
stopped. restart it and make sure it stays started.
Check the service account (LSGroupExpAppPool =>
properties => identity), it should have the latest
password and account should not be set to expire in AD.
Go to Address Book Issues...