zimbraPublicServiceHostname not served consistently
We're running a multi-server environment; version 7.2 Release 2669.
The problem is even though we have zimbraPublicServiceHostname set as well as a virtual host, frequently users from the outside world are redirected to a backend mailbox server upon login. The mailbox server is not directly accessible from the Internet, and traffic is supposed to be handled through the proxy server. We currently have two proxy servers (mta), one mailbox server, ldap server, and two archive servers. The problem began recently after adding a new archive server. All servers are CentOS5 except the new one which is CentOS6. All servers run the same version of zimbra. Browers affected are IE, Chrome, Firefox, Safari, so it doesn't appear to necessarily be a browser problem.
I haven't found anything obvious in any of the log files on any server, but it appears that occasionally, the zimbraPublicServiceHostname is not being used when transmitting data back to clients. These are users trying to log in via the web interface.
Would anyone have any insight as to where and how to begin troubleshooting this?