I tried several things for a couple of days but it didn't work and I had no option but to start from scratch. Now it's working.
I notice one mistake I might have made was to use "SYSTEM_DOMAIN" for "common name" instead of "*.SYSTEM_DOMAIN" when creating the SSL certificate (openssl genrsa -out cf.key 1024).
I also notice some steps in the tutorial are slightly old/outdated w.r.t. to AWS EC2 versioning, configuration ...
I tried the command you sent (don't think it matters now) and it returns "logging_endpoint":"wss://loggregator.SYSTEM_DOMAIN:4443"