Blank pages after upgrade to 18

Dear community,

i updated my NC17, which was running smoothly on a netcup web-hosting instance (with restricted ssh access). Like always i used the online updater. After completing all 12 upgrade steps i returned to the cloud but nothing happened. No HTML was loaded at all, the NC app told me that the cloud is in maintenance mode.
The config file seemed untouched after the upgrade with no version change and maintenance mode set to false. The backup was performed and based on the timestamps the other files were also replaced.

EDIT: I was able to recover the previous NC17 with a backup. I upgraded the instance again, which also resulted in blank pages.

Any help is much appreciated.
Thanks
Jakob

Nextcloud version : 17.0.1.1
PHP version: 7.3.15
System: Webhosting storage
Is this the first time you’ve seen this error?: Y

Steps to replicate it:

  1. Upgrade via online updater to nc 18.0.1

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

<?php
$CONFIG = array (
  'instanceid' => '***',
  'passwordsalt' => '***',
  'secret' => '***',
  'trusted_domains' => 
  array (
    0 => '***',
    1 => '***',
    2 => '***',
    3 => '***',
  ),
  'datadirectory' => '/var/www/vhosts/hosting****.netcup.net/nextcloud/data',
  'dbtype' => 'mysql',
  'version' => '17.0.1.1',
  'overwrite.cli.url' => '***',
  'dbname' => '***nextcloud',
  'dbhost' => '***',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => '****',
  'dbpassword' => '****',
  'installed' => true,
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,
  'updater.secret' => '***',
);