Error after reinstalling migrate modules: mt_rand(): max(-1) is smaller than min(0)

Hi,

after I encountered this problem I uninstalled and reinstalled migrate, migrate_upgrade and migrate_drupal. When trying to "Rerun" import via UI (/upgrade) I get the following errors:
Warning: mt_rand(): max(-1) is smaller than min(0) in Drupal\Core\Database\Database::parseConnectionInfo() (line 217 of core/lib/Drupal/Core/Database/Database.php).
Notice: Undefined offset: 0 in Drupal\Core\Database\Database::parseConnectionInfo() (line 217 of core/lib/Drupal/Core/Database/Database.php).
Notice: Undefined index: driver in Drupal\Core\Database\Database::openConnection() (line 369 of core/lib/Drupal/Core/Database/Database.php).
Notice: Undefined index: driver in Drupal\migrate_upgrade\Form\MigrateUpgradeForm->validateCredentialForm() (line 986 of modules/migrate_upgrade/src/Form/MigrateUpgradeForm.php).

Resolve the issue below to continue the upgrade. Driver not specified for this database connection: upgrade

I have not specified a database called "upgrade". This is what's in my settings.php:

$databases['drupal6_kupf']['default'] = array (
  'database' => 'drupal6_kupf',
  'username' => 'drupal6_kupf',
  'password' => 'secret',
  'prefix' => '',
  'host' => 'localhost',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
);
$databases['default']['default'] = array (
  'database' => 'drupal8_kupf',
  'username' => 'kupf8',
  'password' => 'secret',
  'prefix' => '',
  'host' => 'localhost',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
);

I there a way to change/set the database settings used for the upgrade process?