Navigating to some links, such as Active Session links, within the SG Admin Console presents the error An unspecified problem has occurred. The system has returned a 502 status code. Management Center failed to connect to the device's web console: UNREACHABLE_HOST
Some links retrieve content from the ProxySG appliance's advanced URLs, hosted at the web console port 8082.
Ensure any intermediary devices, such as a firewall, allows Management Center to access the ProxySG appliance over TCP port 8082.
Other 502 status codes for errors CERTIFICATE_UNTRUSTED and PEER_UNVERIFIED are discussed here.