Logging problem 18.0.4

[details=“only mail app writes to nextcloud.log”]

Fresh install of Raspbian and Nextcloud
Nextcloud version (18.0.4):
Operating system and version Raspbian Buster
Apache or nginx version (Apache 2.4.38):
PHP version (7.3):

The issue you are facing:
logging configuration seems to be o.k. but only the mail app writes to the logfile.
The nextcloud.log does not contain entries from any other entity, loglevevel is 1.
Are there any other logfiles I can check for? What is the difference between the mail app and other apps regarding the logging mechanism?

config.php:

<?php
$CONFIG = array (
  'instanceid' => 'xxx',
  'passwordsalt' => 'x+x',
  'secret' => 'x/x+x',
  'trusted_domains' =>
  array (
    0 => 'localhost',
    ),
  'datadirectory' => '/var/www/html/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '18.0.4.2',
  'overwrite.cli.url' => 'http://localhost/nextcloud',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'xxx',
  'dbpassword' => 'xxx',
  'installed' => true,
  'mail_from_address' => 'xxx',
  'mail_smtpmode' => 'smtp',
  'mail_sendmailmode' => 'smtp',
  'mail_domain' => 'xxx.de',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtphost' => 'xxx',
  'mail_smtpport' => '587',
  'mail_smtpname' => 'xxx',
  'mail_smtppassword' => 'xxx',
  'mail_smtpsecure' => 'tls',
  'maintenance' => false,
  'loglevel' => 1,
  'log_type' => 'file',
  'logtimezone' => 'Europe/Berlin',
  'updater.release.channel' => 'stable',
);

Database entries for logreader from mariadb:

('logreader', 'enabled', 'yes'),
('logreader', 'installed_version', '2.3.0'),
('logreader', 'levels', '01111'),
('logreader', 'live', '1'),
('logreader', 'relativedates', ''),
('logreader', 'types', '');

Does anyone have an idea?