May be you don’t have proper SSL certificates. How can you tell more details about how exactly you using your lexmachina.bydiscourse.com and citadel-discussion.lexmachina websites?
Are you just pointed citadel-discussion.lexmachina CNAME into lexmachina.bydiscourse.com? I tried to open this url lexmachina.bydiscourse.com on my browser and am unable to reach it.