By default on SGOS v5.5, an enhanced HTTP feature called 'HTTP Network Failure Transparency' is enabled. Therefore, the ProxySG serves a 'TCP_ERROR' exception page when an upstream server connection error occurs.
If you would like to disable this feature, so that an exception is not served to clients on your network when a connection failure occurs, log into the ProxySG CLI and enter
SG#(config) http no exception-on-network-error
The ProxySG behavior will now be the same as v5.4 and earlier versions.