I just found this error message in the Apache's log (apache2/logs/error_log file)
[Tue Sep 17 04:55:16.835908 2019] [proxy_fcgi:error] [pid 1995:tid 139696562763520] (20014)Internal error (specific information not available): [client 18.104.22.168:45282] AH01075: Error dispatching request to : (reading input brigade)
It seems that the PHP-FPM server is not running, can you verify that?
sudo /opt/bitnami/ctlscript.sh status
If there is any service stopped, please start it
sudo /opt/bitnami/ctlscript.sh start