If a secondary domain runs on the same server as a primary, then there is no error. However, when you try to run a Validate or a Rebuild Database, you get SSL certificate complains "javax.net.ssl.SSLException: readHandshakeRecord (null)".
Similarly, when you are connect with gwadmin console to a secondary domain and want to perform any operations on the primary domain, you get also SSL certificate complains.System certificates were regenerated as well as any certificate on agents but the problem still exists.
This has been fixed and the fix is available in FTF builds for GW 18.2.1 136736 or newer. You will need to ask our support center to obtain any newer FTF builds.