Security: "__Host-Prefix", how to fix?

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/etc/apache2/sites-enabled/
├── 000-default.conf -> …/sites-available/000-default.conf
└── nextcloud.conf -> …/sites-available/nextcloud.conf

Debian Stretch
PHP 7.0.27
SQLite 3.16.2
Apache 2.4.25
NC dir /var/www/html/nextcloud