Django - Unable to open database file (sqlite3.OperationalError)

I am developing a small app with the django framework and when trying to serve it via apache2 I get the following error:

(sqlite3.OperationalError) unable to open database file (Background on this error at: http://sqlalche.me/e/e3q8)

My settings.py entry for the database looks like that:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3')
    }
}

The db file exists and has the following rights given by chmod:

root@myVPS:/var/www/lab# ls -l db.sqlite3
total 30236
-rwxrwxr-x 1 www-data www-data  3018568 Jul  1 22:14 db.sqlite3

The root folder /var/www/lab:

drwxrwxr-x 8 www-data www-data 4096 Jul 2 01:15 lab

I read that it might be a problem with the rights but I can't seem to find what I did wrong.

My Apache2 config file:

<VirtualHost *:80>
        ServerAdmin webmaster@localhost
        ServerName sub.domain.com
        ServerAlias sub.domain.com
        DocumentRoot /var/www/lab
        <Directory /var/www/lab>
                Options FollowSymLinks
                AllowOverride All
        </Directory>

        ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
        <Directory "/usr/lib/cgi-bin">
                AllowOverride None
                Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
                Order allow,deny
                Allow from all
        </Directory>

        ErrorLog ${APACHE_LOG_DIR}/error.log

        # Possible values include: debug, info, notice, warn, error, crit,
        # alert, emerg.
        LogLevel warn

        CustomLog ${APACHE_LOG_DIR}/access.log combined

        Alias /static /var/www/lab/static
        <Directory /var/www/lab/static>
                Require all granted
        </Directory>

        <Directory /var/www/lab/chatbot>
                <Files wsgi.py>
                        Require all granted
                </Files>
        </Directory>

        WSGIScriptAlias / /var/www/lab/chatbot/wsgi.py
        WSGIDaemonProcess django_app python-path=/var/www/lab python-home=/var/www/lab/venv
        WSGIProcessGroup django_app

</VirtualHost>

I'm running Debian 8 with Python 3.5.3 and Django 2.2.3.

from Django - Unable to open database file (sqlite3.OperationalError)