In an open managed ADN environment, ADN peer gets an error "SSL Connect failed" while connecting to a ADN manager. This can especially happen if the existing ADN manager is replaced with a new unit.
Under Configuration -> ADN -> General -> General [Tab] -> SSL Connect failed (error:0x3EC) appears when clicking on "Reconnect to Managers".
The device authentication and authorization is enabled at the ADN manager. In such cases, the manager validates the authentication by verifying the ProxySG appliance's peer ID before allowing it to join the ADN. The peer ID is determined by fetching the hardware serial number from the CommonName (CN) in the subject field of the appliance certificate. In addition, the manager has to manually accept the device ID under Configuration -> ADN -> Manager -> Pending Peers to complete the authorization process.
Check the following at the ADN peer:
a) If the appliance certificate is used as the SSL Device profile, ensure it is present in the device. For more information on appliance certificate and how to obtain a certificate, refer the KB article: TECH244738
b) Under Configuration -> ADN -> General -> Device Security -> Change the Primary Manager Device ID to the new serial number of the "Primary ADN manager" or click on "Retreive Manager IDs" to automatically fetch the peer id from manager.