Read the error
Your error message clearly states that it's not configured to allow SSL connections on port 7071:
HTTP 502 Proxy Error - The specified Secure Sockets Layer (SSL) port is not allowed. ISA Server is not configured to allow SSL requests from this port. Most Web browsers use port 443 for SSL requests. (12204)
Internet Security and Acceleration Server
Until you fix that problem, there's nothing we can do to help you - the problem is in your proxy server configuration, not in the zimbra system.
to have ssl go on non standard ports, you need to change the com.
or I think there is a script on this site for 2000
Mind you, I'm trying from across the internet to access my server for admin.
I have ISA2004 where my client is and a DSL router at the other end with the zimbra server NATed, with connections on 7071 port redirected to the server. I'm trying to work out the error that appears once you've fixed the one you're getting: failed connection attempt (status code 10061)