1. Set the proxy configuration / Services / Proxy services / HTTPS / port 443 / Proxy setting as TCP Tunnel, (It must be TCP tunnel, not SSL proxy), Enable Detect protocol, All source IP, All destination IP, port 443, Intercept.
2. In Proxy Setting / General / Enable TCP Tunnel requests when a protocol error detected. This option only available after SGOS 5.5.
Note as online document or proxy Admin guide:
For the SSL proxy, the Tunnel on Protocol Error option applies when non-SSL traffic arrives at the SSL port (443 by default). A common scenario that causes this is having peer-to-peer applications (viz, Skype, BitTorrent, Gnutella, older AOL-IM and eMule) configured to enable port 443 for peer-to-peer traffic without SSL set as the transport protocol. A ProxySG transparently intercepting all 443 traffic cannot process these connections, rendering the application unusable.