Initially, the link to phpMyAdmin in the database table was insecure (e.g. http://
).
However, I had been connecting to aaPanel securely, by assigning a domain, and then installing a certificate, from Let's Encrypt.
I followed instructions to secure phpMyAdmin, by upgrading the version, and enabling SSL. I also opened the port, 887.
If I connect to phpMyAdmin through the secure port, the connection is encrypted, but the browser window page shows the error page for HTTP 503.