I have setup zimbra on an internal server, and it is workiing fine. It uses our local ldap server to authenticate the users. However, I want to be able to access the server from the external world. I have a firewall protecting the network.

Zimbra is running on fc4.

I have a server running apache in the dmz that forwards the webmail requests through the fw via port 18880. When I am outside my network and go to http://webmail... The zimbra login page comes up. However, when I go to login, I get an error message:
An unknown application error has occurred. Please correct any errors and retry.

I can see the traffic using tcpdump on the zimbra server, and it appears to be fine. I cannot find any error messages in the log files as to why the authentication might be failing.

My external apache configuration is:
<VirtualHost x.x.x.x:80>
ServerName webmail.x.com
RedirectPermanent / http://webmail.x.com/
ProxyPass / http://x.x.x.x:18880/
ProxyPassReverse / http://x.x.x.x:18880/
ProxyVia Off

Is there perhaps a proxy config problem perhaps?

I have tried a number of different apache configs, but I see to get the same result each time.

Any help would be appreciated, and any additional information I can provide.