Do the following to enable clean URL in Debian 8:
sed -i '/var\/www\//,/Directory>/ s/AllowOverride None/AllowOverride All/' /etc/apache2/apache2.conf
service apache2 restart
Transfer your Drupal instance from 1 system to another 1. Then, on the latter system, it doesn't work(e.g. page not found, can't log in, url screw up, etc).
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