SGAC (SG Admin Console)
Management Center's ssl-context was change to validate the management devices' HTTPS-Console certificate (e.g Device-Communication ssl-context was modified from "Global Default" to "Default")
Manage device (e.g. SG or ASG) is presenting a certificate (self signed or signed) that Management Center unable to verify.
An MITM (man-in-the-middle) that intercepts SSL/TLS on port 8082
If you don't intend to verify Manage Devices Certificate, set the SSL-Context back to "Global Default"
IF none of the above applies to you, check your network for potential MITM that may intercept 8082 between Management Center and SG/ASG
When using SGAC, Management Center has the potential to use both ssl-console (port 22) and https-console (port 8082) to communicate with ProxySG or ASG