UCS with Nextcloud-Docker ... Internal Server Error after Installation

I just installed Nextcloud for testing purposes.
I run UCS and installed Nextcloud from the UCS-App-Center.
Installation went fine, no errors. But If I call the Nextcloud-Startpage I get:

Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

But I can’t even find a logfile…

This is the conf

$CONFIG = array (
  'passwordsalt' => '***',
  'secret' => '***',
  'trusted_domains' =>
  array (
    0 => 'subdomain.domain.tld',
    1 => '10.10.10.21',
  ),
  'datadirectory' => '/var/lib/univention-appcenter/apps/nextcloud/data/nextcloud-data',
  'dbtype' => 'pgsql',
  'version' => '14.0.4.2',
  'overwrite.cli.url' => 'https://subdomain.domain.tld/nextcloud',
  'dbname' => 'nextcloud',
  'dbhost' => '172.17.42.1',
  'dbport' => '5432',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'nextcloud',
  'dbpassword' => '***',
  'installed' => true,
  'instanceid' => '***',
  'updatechecker' => 'false',
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'overwriteprotocol' => 'https',
  'overwritewbroot' => '/nextcloud',
  'htaccess.RewriteBase' => '/nextcloud',
  'ldapIgnoreNamingRules' => false,
  'ldapProviderFactory' => 'OCA\\User_LDAP\\LDAPProviderFactory',
  'trusted_proxies' =>
  array (
    0 => '172.17.42.1',
  ),
  'loglevel' => 2,
  'maintenance' => true,
);

sensible Data is replaced by “***” or “subdomain.domain.tld”

Any Ideas? the IP for the Database could be wrong?.. There is no such IP with our server…

There are three UCS-Servers … Should I fill in the UCS-Master here or the machine where nextcloud is installed on?