Reverse Proxy not running on HTTPS Apache Server - apache

Question

Created a Hello_World NodeJS App.
The app is accessible at http://<DOMAIN_NAME>/app & http://<DOMAIN_NAME>:3000
but when accessed via https://<DOMAIN_NAME>/app it returns an error.
http://<SERVER_IP>:3000 also works & https:<SERVER_IP>:3000 returns an error
sudo apachectl -S
OUTPUT :
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message
VirtualHost configuration:
*:80 okoyama.in (/etc/apache2/sites-enabled/okoyama.in.conf:1)
*:443 okoyama.in (/etc/apache2/sites-enabled/okoyama.in.conf:45)
ServerRoot: "/etc/apache2"
Main DocumentRoot: "/var/www/html"
Main ErrorLog: "/var/log/apache2/error.log"
Mutex default: dir="/var/run/apache2/" mechanism=default
Mutex mpm-accept: using_defaults
Mutex watchdog-callback: using_defaults
Mutex proxy-balancer-shm: using_defaults
Mutex rewrite-map: using_defaults
Mutex ssl-stapling-refresh: using_defaults
Mutex ssl-stapling: using_defaults
Mutex proxy: using_defaults
Mutex ssl-cache: using_defaults
PidFile: "/var/run/apache2/apache2.pid"
Define: DUMP_VHOSTS
Define: DUMP_RUN_CFG
User: name="www-data" id=33
Group: name="www-data" id=33
<VirtualHost *:80>
ServerAdmin admin#okoyama.in
ServerName okoyama.in
ServerAlias www.okoyama.in
DocumentRoot /var/www/okoyama.in/public_html
ProxyPreserveHost On
ProxyPass /app http://localhost:3000/
ProxyPassReverse /app http://localhost:3000/
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
<IfModule mod_ssl.c>
<VirtualHost *:443>
DocumentRoot /var/www/okoyama.in/public_html
ServerName okoyama.in
SSLEngine on
SSLCertificateFile /etc/ssl_install/certificate.crt
SSLCertificateKeyFile /etc/ssl_install/private.key
SSLCertificateChainFile /etc/ssl_install/ca_bundle.crt
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
</IfModule>
The above code has Reverse Proxy enabled for HTTP. HTTPS works normally. I have removed the Reverse Proxy section from 443 Port for now to make the working code cleaner.
Multiple variations of solutions suggested on various platforms have been tried in Port 443 section but none worked until now.
The included turning on SSLProxyEngine & a similar Proxy method as mentioned in the HTTP section.
Suggest possible solution. I will retry & post for clarity.