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.