I am having a complicated issue, which I can't figure out. There is a solution to this matter by modifying the 000-default file in /etc/apache2/sites-enabled.
The AllOverride line must be left with the following keyword None.
<Directory /var/www/>
Options Indexes FollowSymLinks MultiViews
AllowOverride None
Order allow,deny
allow from all
</Directory>
Is there any other way to enable clean URLs on a Debian server for Drupal 7?