502 Bad Gateway after changing listening port to :443

Mattermost Version: 9.6.0 (Bitnami VM distro).

This is an out-of-the box install using Bitnami Team Edition via VMware import.
Hosted locally on our server, with ports 80, 443, 8065 open to outside with a public-facing domain.
Internally, the same domain is set in DNS pointing to the local IP.

Running nslookup on the internal IP returns to the correct domain of the mattermost server.
Running nslookup on the external DNS IP returns the correct domain of the mattermost server.

I have the SSL cert installed, and it is recognized in the browser as the one I installed with the correct CA, but says ‘not secure’ on the address bar of Edge.

In System Console > Environment > Web Server, when I change Listen Address to :443 and Save and reboot, then my web browser gives the 502 error. I have to restore the config.json from backup and reboot bitnami to get Mattermost working, but the SSL is still not secure.

How do I debug this bad gateway issue?