The security certificate presented by this website was not issued by a trusted certificate authority.

If you are running several namebased SSL VirtualHosts with mod_GnuTLS, you need to provide the adequate intermediate certificate to check that your SSL certificates chains are valid. With mod_GnuTLS, it is quite tricky to set it up. You do not have any options. The only way to make it work is to do as follow :

For a Debian-based distro :

root@localhost:~# cd /etc/ssl/certs

root@localhost:~#cat your_SSL_intermediate_certificate_provider.pem your_ssl_website_certificate.crt > your_new_ssl_certificate.crt

In the Apache config file for your SSL website, add the following line :

GnuTLSCertificateFile /etc/ssl/certs/your_new_ssl_certificat_site.crt

root@localhost:~#/etc/init.d/apache2 reload

More infos about GnuTLS

Invalid command ‘Order’, perhaps misspelled or defined by a module not included in the server configuration failed!

By mistake, I deleted all the content of the directory /etc/apache2/mods-enabled/.
I was unable to restart the Apache server.
I got the following error :

root@localhost:~# service apache2 restart
Restarting web server: apache2Syntax error on line 143 of /etc/apache2/apache2.conf:
Invalid command ‘Order’, perhaps misspelled or defined by a module not included in the server configuration failed!

A similar problem has already been met and registered as a Debian bug :
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=392349

The command ‘Order’ in Apache configuration file is managed through authz_host module.
Following their solution, I reloaded the modules below:

root@localhost:~# a2enmod authz_host
root@localhost:~# a2enmod mime
root@localhost:~# a2enmod alias
root@localhost:~# a2enmod setenvif

and then I restarted the Apache server successfully :

root@localhost:~# service apache2 restart

For more information about Apache modules, please have a look at their official documentation :
http://httpd.apache.org/docs/2.2/