During installation both the MSS and HA Cloud servers have exchanged and trusted their certificates. When you add additional MSS servers, their certificates also need to be trusted.
To setup trust MSS and the session servers
Trust the new MSS server by importing the MSS certificate into the session server’s truststore. See Import a certificate into the session server’s truststore
The new MSS needs to trust each of the session servers.
Copy the certificate to %ProgramData%\Micro Focus\Mss\MssData\certificates .
As the administrator, log into MSS.
From the left panel, click Configure Settings > Trusted Certificates.
Select Management and Security Server. The list contains the certificates that are trusted by MSS.
Click Import to add the session server certificate to the list.
Repeat this process for each session server.