A component of the Active Directory system is currently in transition and as a result no single version of the telephone directory will consistently be able to authenticate you.
Therefore, please first try:
https://webapp.services.coventry.ac.uk/AddressBook1.03-dc01/0/Main.html
and if this gives an error, then try:
https://webapp.services.coventry.ac.uk/AddressBook
As always, please use the following for creating links and bookmarks. The link currently points to this page, but will be moved directly to the telephone directory once the issue is resolved.
http://www.coventry.ac.uk/addressbook
If you still have problems, please report these to the service desk in the normal way.