http://cs.thefoleyhouse.co.uk/blogs/karl/archive/2008/12/13/communicator-2007-fails-to-download-corporate-address-book.aspxIn my case, this was caused by https certificate problems. The certificate had been generated during the install, but had not been applied to the IIS server. Can also be caused by firewall issues with ports or IIS MIME issues for dabs (application\dabs)enCommunityServer 2.1 SP3 (Build: 20423.1)http://cs.thefoleyhouse.co.uk/blogs/karl/archive/2008/12/13/communicator-2007-fails-to-download-corporate-address-book.aspx#1096Mon, 22 Dec 2008 17:09:53 GMT72ff1f2b-f56d-431e-a40a-3b65004deded:1096Karl<p>You may also need to run the adress book sync manually if the server has only just been installed:</p> <p>c:\Program Files\Micrsoft Communications Server 2007\Server\Core\ABServer.exe -syncNow</p> <p>Thanks to:</p> <p><a rel="nofollow" target="_new" href="http://social.microsoft.com/forums/en-US/communicationsserveruser/thread/c7e65f29-0c3b-4371-9482-4a5bb875129d/">http://social.microsoft.com/forums/en-US/communicationsserveruser/thread/c7e65f29-0c3b-4371-9482-4a5bb875129d/</a></p>