The Secure Proxy's NGINX /usr/local/nginx/logs/controller.log file has "Failed to create SSL Connection" on the javax.net.ssl.SSLHandshakeException. This SSL handshake error is preventing the Secure Proxy server from registering to the Symantec Mobility Front End.
This same error may also prevent email sync and push functionality while communicating between the EAS/EWS front ends. See the note below regarding how to use these same steps to resolve other SSL Java related connectivity issues.
The SSL certificate installed on the network resource is not trusted by Java.
Note: Several things can cause an Secure Proxy server to not be able to register to a Mobility Suite Front End (FE) server or lose it's connectivity thereto. First verify that the server has direct outbound access over TCP 443 to the fully qualified domain name (FQDN) of the FE. Also confirm that a local administrative account is being used to register the Secure Proxy to the FE. Steps 5 and 6 may be repeated substituting the internal CAS/EAS/EWS server FQDN for the Mobility FQDN in the keytool command if having this connectivity issue while attempting to send/receive email or register the impersonation account.