su - zimbra
zmprov mcf zimbraWebClientLogoutURL
http://www.zimbra.com
or
zmprov md domain.com zimbraWebClientLogoutURL
http://www.zimbra.com
I believe for the zimbraWebClientLogoutURL to work, you might need to create a virtual domains for all possible redirects.
zmprov md domain.com +zimbraVirtualHostname
http://www.zimbra.com
(You can do this from the admin console gui as well.)
It should come right up, but before posting that it doesn't try a zmmailboxdctl restart.