Ssl – Address already in use, could not bind to address [::]:443

I am trying to set up SSL within my ubuntu 14.04 server. After generating my self-signed RSA certificate and setting up all necessary configuration, my server failed to start. All my /var/log/apache2/error.log, /var/log/apache2/access.log and /var/log/apache2/other_vhosts_access.log are empty however I am getting this message from the console :

server failure screenshot

My /etc/apache2 directory tree :

/etc/apache2
|---> ... other files
|---> ssl
        |---> server.key
        |---> server.crt
|---> ports.conf
|---> sites-available
        |
        |---> default-ssl.conf
|---> sites-enabled
        |
        |---> default-ssl.conf

Checking if the the default configuration is enabled :

$sudo a2ensite default-ssl.conf
Site default-ssl already enabled

The content of default-ssl.conf :

$cat default-ssl.conf
<IfModule mod_ssl.c>
    <VirtualHost _default_:443>
            ServerAdmin webmaster@localhost
            ServerName NetSec

            DocumentRoot /var/www/html

            # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
            # error, crit, alert, emerg.
            # It is also possible to configure the loglevel for particular
            # modules, e.g.
            #LogLevel info ssl:warn

            ErrorLog ${APACHE_LOG_DIR}/error.log
            CustomLog ${APACHE_LOG_DIR}/access.log combined

            # For most configuration files from conf-available/, which are
            # enabled or disabled at a global level, it is possible to
            # include a line for only one particular virtual host. For example the
            # following line enables the CGI configuration for this host only
            # after it has been globally disabled with "a2disconf".
            #Include conf-available/serve-cgi-bin.conf

            #   SSL Engine Switch:
            #   Enable/Disable SSL for this virtual host.
            SSLEngine on

            #   A self-signed (snakeoil) certificate can be created by installing
            #   the ssl-cert package. See
            #   /usr/share/doc/apache2/README.Debian.gz for more info.
            #   If both key and certificate are stored in the same file, only the
            #   SSLCertificateFile directive is needed.
            #SSLCertificateFile      /etc/ssl/certs/ssl-cert-snakeoil.pem
            SSLCertificateKeyFile /etc/apache2/ssl/server.key

            #   Server Certificate Chain:
            #   Point SSLCertificateChainFile at a file containing the
            #   concatenation of PEM encoded CA certificates which form the
            #   certificate chain for the server certificate. Alternatively
            #   the referenced file can be the same as SSLCertificateFile
            #   when the CA certificates are directly appended to the server
            #   certificate for convinience.
            SSLCertificateChainFile /etc/apache2/ssl/server.crt

            #   Certificate Authority (CA):
            #   Set the CA certificate verification path where to find CA
            #   certificates for client authentication or alternatively one
            #   huge file containing all of them (file must be PEM encoded)
            #   Note: Inside SSLCACertificatePath you need hash symlinks
            #                to point to the certificate files. Use the provided
            #                Makefile to update the hash symlinks after changes.
            #SSLCACertificatePath /etc/ssl/certs/
            #SSLCACertificateFile /etc/apache2/ssl.crt/ca-bundle.crt

            #   Certificate Revocation Lists (CRL):
            #   Set the CA revocation path where to find CA CRLs for client
            #   authentication or alternatively one huge file containing all
            #   of them (file must be PEM encoded)
            #   Note: Inside SSLCARevocationPath you need hash symlinks
            #                to point to the certificate files. Use the provided
            #                Makefile to update the hash symlinks after changes.
            #SSLCARevocationPath /etc/apache2/ssl.crl/
            #SSLCARevocationFile /etc/apache2/ssl.crl/ca-bundle.crl

            #   Client Authentication (Type):
            #   Client certificate verification type and depth.  Types are
            #   none, optional, require and optional_no_ca.  Depth is a
            #   number which specifies how deeply to verify the certificate
            #   issuer chain before deciding the certificate is not valid.
            #SSLVerifyClient require
            #SSLVerifyDepth  10

            #   SSL Engine Options:
            #   Set various options for the SSL engine.
            #   o FakeBasicAuth:
            #        Translate the client X.509 into a Basic Authorisation.  This means that
            #        the standard Auth/DBMAuth methods can be used for access control.  The
            #        user name is the `one line' version of the client's X.509 certificate.
            #        Note that no password is obtained from the user. Every entry in the user
            #        file needs this password: `xxj31ZMTZzkVA'.
            #   o ExportCertData:
            #        This exports two additional environment variables: SSL_CLIENT_CERT and
            #        SSL_SERVER_CERT. These contain the PEM-encoded certificates of the
            #        server (always existing) and the client (only existing when client
            #        authentication is used). This can be used to import the certificates
            #        into CGI scripts.
            #   o StdEnvVars:
            #        This exports the standard SSL/TLS related `SSL_*' environment variables.
            #        Per default this exportation is switched off for performance reasons,
            #        because the extraction step is an expensive operation and is usually
            #        useless for serving static content. So one usually enables the
            #        exportation for CGI and SSI requests only.
            #   o OptRenegotiate:
            #        This enables optimized SSL connection renegotiation handling when SSL
            #        directives are used in per-directory context.
            #SSLOptions +FakeBasicAuth +ExportCertData +StrictRequire
            <FilesMatch "\.(cgi|shtml|phtml|php)$">
                            SSLOptions +StdEnvVars
            </FilesMatch>
            <Directory /usr/lib/cgi-bin>
                            SSLOptions +StdEnvVars
            </Directory>

            #   SSL Protocol Adjustments:
            #   The safe and default but still SSL/TLS standard compliant shutdown
            #   approach is that mod_ssl sends the close notify alert but doesn't wait for
            #   the close notify alert from client. When you need a different shutdown
            #   approach you can use one of the following variables:
            #   o ssl-unclean-shutdown:
            #        This forces an unclean shutdown when the connection is closed, i.e. no
            #        SSL close notify alert is send or allowed to received.  This violates
            #        the SSL/TLS standard but is needed for some brain-dead browsers. Use
            #        this when you receive I/O errors because of the standard approach where
            #        mod_ssl sends the close notify alert.
            #   o ssl-accurate-shutdown:
            #        This forces an accurate shutdown when the connection is closed, i.e. a
            #        SSL close notify alert is send and mod_ssl waits for the close notify
            #        alert of the client. This is 100% SSL/TLS standard compliant, but in
            #        practice often causes hanging connections with brain-dead browsers. Use
            #        this only for browsers where you know that their SSL implementation
            #        works correctly.
            #   Notice: Most problems of broken clients are also related to the HTTP
            #   keep-alive facility, so you usually additionally want to disable
            #   keep-alive for those clients, too. Use variable "nokeepalive" for this.
            #   Similarly, one has to force some clients to use HTTP/1.0 to workaround
            #   their broken HTTP/1.1 implementation. Use variables "downgrade-1.0" and
            #   "force-response-1.0" for this.
            BrowserMatch "MSIE [2-6]" \
                            nokeepalive ssl-unclean-shutdown \
                            downgrade-1.0 force-response-1.0
            # MSIE 7 and newer should be able to use keepalive
            BrowserMatch "MSIE [17-9]" ssl-unclean-shutdown

    </VirtualHost>  
</IfModule>

The content of ports.conf file :

$cat /etc/apache2/ports.conf

Listen 80
Listen 443

<ifModule ssl_module>
    Listen 443
</ifModule>

<ifModule mod_gnutls.c>
    Listen 443
<ifModule>

The symbol link pointing to default-ssl.conf is well established :

$ls -l /etc/apache2/sites-enabled/default-ssl.conf
lrwxrwxrwx 1 root root 35 Jun 25 15:42 default-ssl.conf -> ../sites-available/default-ssl.conf

At the end I would like to stress that I have checked the completeness of my private key/certificate and the check does hold. I have followed actually the steps mentioned in this tutorial in order to generate my self-signed server certificate.

Any help will be more than appreciated. Thank you.