Upgrade -> 12.0.1 fails, "An error occurred."

My environment:
Nextcloud version (eg, 10.0.2): See title.
Operating system and version (eg, Ubuntu 16.04): Debian, don’t know which one (don’t have sudo rights, lsb-release not existing, many system tools don’t work, only basic operations avaiable).
Apache or nginx version (eg, Apache 2.4.25): Nginx, don’t know which version
PHP version (eg, 5.6): 7.0.21
Is this the first time you’ve seen this error?: Yes.

Can you reliably replicate it? (If so, please outline steps):

  • Disable maintenance mode manually in config file
  • open nextcloud URL, hit button “Start update”

The issue you are facing:
When I click on the update button in my nextcloud instance, this is theoutput:

Preparing update
Set log level to debug
Turned on maintenance mode
Updating database schema
Updated database
Checking for update of app "activity" in appstore
An error occurred.

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

$CONFIG = array (
  'instanceid' => '...',
  'passwordsalt' => '...',
  'secret' => '...',
  'trusted_domains' => 
  array (
    0 => 'cloud.URL',
  'datadirectory' => '/var/www/vhosts/.../private/nextclouddata',
  'overwrite.cli.url' => 'URL',
  'dbtype' => 'mysql',
  'version' => '',
  'dbname' => '...',
  'dbhost' => '...',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => '...',
  'dbpassword' => '...',
  'logtimezone' => 'UTC',
  'installed' => true,
  'theme' => '',
  'loglevel' => 0,
  'maintenance' => true,
  'updater.secret' => '...',

The output of your Apache/nginx/system log in /var/log/____:
-> No error is thrown during the installation progress.

I cannot run the occ CLI tool. Update should work using the web anyhow. Any help?

logs are in data/nextcloud.log

OK, meanwhile I reinstalled 12.0.1 completely. Thanks for your help though.