I noticed once the following error:
AH00526: Syntax error on line 26 of /etc/apache2/sites-enabled/000-default.conf:
Invalid command ‘AuthGroupFile’, perhaps misspelled or defined by a module not included in the server configuration
Action ‘configtest’ failed.
The Apache error log may have more information.
It is solved simply by activating the module:
sudo a2enmod authz_groupfile
Restart apache2 to apply the changes:
sudo service apache2 restart