How to diagnose an “AH00111: Config variable is not defined” when the system env var is defined

I am setting up an Apache 2.4.7 server on a Ubuntu 14.04.5 LTS machine, with the intention of replacing another instance. Basically, the new instance is a pristine Ubuntu 14.4 install, with only Apache installed. I have copied and pasted my entire /etc/apache2 folder from the old machine to the new one.

I am getting this error:

AH00111: Config variable ${PORTAL_SERVER} is not defined

There are other questions about this error, such as this one or this one. However, the solutions or answers leave me puzzled because on both machines:

• PORTAL_SERVER is defined in /etc/environment
• echo $PORTAL_SERVER gives me the expected value
• printenv lists the environment variable with the expected value
• the contents of /etc/apache2 is exactly the same, in particular, that of /etc/apache2/envvars.

Only, on the new machine, sudo apachectl -V gives me an AH00111 error.

The service gets started, but obviously, not as I wanted it.

sudo apachectl graceful gives me the same message, which feels normal. sudo service apache2 restart does the same.

My question is why doesn't the apache user "see" the environment variable?

EDIT:
Here is the entire custom configuration (in a single conf file enabled in sites-enabled), where the ... represent the multiple ProxyPass directives that constitute the rest of the file. The standard Apache configuration files were not edited.

<VirtualHost *:443>
        # This virtual host will answer requests on port 443 (https) for the domain defined hereafter
        # NOTE: The PORTAL_SERVER environment variable is defined in /etc/environment.
        ServerName ${PORTAL_SERVER}

        # Define error page 503 (site under maintenance) : reached when a server doesn't answer ajp requests
        DocumentRoot /var/www
        Alias "/errors" "/var/www/errors"
        <Directory  "/var/www/errors/">
                Options FollowSymLinks MultiViews
                order deny,allow
                allow from all
        </Directory>
        ErrorDocument 503 /errors/error-503.html

        # Redirect https://${PORTAL_SERVER}/ on https://${PORTAL_SERVER}/portal/
        Redirect /index.html https://${PORTAL_SERVER}/portal/

        # Activate SSL and define certificat
        SSLEngine on
        SSLProtocol all
        SSLCertificateFile    /etc/ssl/certs/ssl_certificate.crt
        SSLCertificateKeyFile /etc/ssl/private/ssl_server.key
        SSLCertificateChainFile /etc/ssl/certs/IntermediateCA.crt

        # Configure mod_proxy
        ProxyRequests Off
        ProxyPreserveHost Off
        # Default timeout before ajp requests are considered as timed out
        ProxyTimeout 10

        ...
        ...

</VirtualHost>