New/first installation: 403 Forbidden

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Nextcloud version (eg, 12.0.2): 13.0.2
Operating system and version (eg, Ubuntu 17.04): 16.04.4
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.18 (Ubuntu)
PHP version (eg, 7.1): 7.0.28-0ubuntu0.16.04.1

The issue you are facing:
After installation the website responds with 403 Forbidden

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Enable let’s encrypt certificates and force ssl: https://certbot.eff.org/lets-encrypt/ubuntuxenial-apache
  2. Follow https://docs.nextcloud.com/server/13/admin_manual/installation/source_installation.html#example-installation-on-ubuntu-16-04-lts-server

The output of your Nextcloud log in Admin > Logging:

N/A

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'passwordsalt' => '...',
  'secret' => '...',
  'trusted_domains' =>
  array (
    0 => 'localhost',
  ),
  'datadirectory' => '/home/.../apache/nextcloud/data',
  'overwrite.cli.url' => 'http://www..../nextcloud',
  'htaccess.RewriteBase' => '/nextcloud',
  'dbtype' => 'mysql',
  'version' => '13.0.2.1',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'oc_admin',
  'dbpassword' => '...',
  'installed' => true,
  'instanceid' => '...',
);

The output of your Apache/nginx/system log in /var/log/____:

77.14.*** - - [29/Apr/2018:10:05:18 +0200] "GET /nextcloud HTTP/1.1" 403 728 "-" "Mozilla/5.0 (***"

This is correctly set:

chown -R www-data:www-data