Upgrade NC21 to NC22.0 -> "file_exists(): open_basedir"

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Nextcloud version (eg, 20.0.5): 22.0
Operating system and version (eg, Ubuntu 20.04): debian buster
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.38-3+deb10u5
PHP version (eg, 7.4): 7.4 + 8.0 tried

The issues:

1.) In the admin-panel “users” I cannot create a group. After klicking on “add group” - nothing happens.

2.) If I check “logging”, for each pressing “add group” there is an php-Error shown up, please see below.

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

The output of your Nextcloud log in Admin > Logging:

[PHP] Error: file_exists(): open_basedir restriction in effect. File(/templates/) is not within the allowed path(s): 
(/var/www/clients/client1/web2/supercloud:/var/www/clients/client1/web2/private:/var/www/clients/client1/web2/tmp:/var/www/supercloud.products4more.at/supercloud:/srv/www/supercloud.products4more.at/supercloud:/usr/share/php5:/usr/share/php:/tmp:/usr/share/phpmyadmin:/etc/phpmyadmin:/var/lib/phpmyadmin:/dev/random:/dev/urandom) at /var/www/clients/client1/web2/supercloud/nextcloud/lib/private/Template/Base.php#68

GET /nextcloud/settings/user
from xxx.75.246.xxx by adminuser at 2021-07-18T11:51:25+00:00

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

<?php
$CONFIG = array (
  'instanceid' => 'oc8eok1jl6nb',
  'passwordsalt' => '**CENSORED**',
  'secret' => '**CENSORED**',
  'trusted_domains' => 
  array (
    0 => 'supercloud.**CENSORED**.at',
  ),
  'datadirectory' => '/var/www/clients/client1/web2/supercloud/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '22.0.0.11',
  'overwrite.cli.url' => 'https://supercloud.**CENSORED**.at/nextcloud',
  'htaccess.RewriteBase' => '/nextcloud',
  'default_phone_region' => 'AT',
  'dbname' => 'c1_clouddb',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'c1_klaud',
  'dbpassword' => '**CENSORED**',
  'installed' => true,
  'mail_smtpmode' => 'smtp',
  'mail_smtpsecure' => 'tls',
  'mail_sendmailmode' => 'smtp',
  'mail_from_address' => 'office',
  'mail_domain' => '**CENSORED**.at',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtphost' => 'tesoro.**CENSORED**.at',
  'mail_smtpname' => 'office@**CENSORED**.at',
  'mail_smtppassword' => '**CENSORED**!',
  'maintenance' => false,
  'updater.secret' => '$2y$10$gHv7m5jPjQBKsTCX/brqC.EUGBgaQMd**CENSORED**.',
  'theme' => '',
  'loglevel' => 2,
  'updater.release.channel' => 'stable',
);

Any help out there? Thanks in advance!