Not able to login after upgrade to nextcloud 15.0.4

I am not able to login into nextcloud 15.0.4 after upgrade. The apache2 error log (/var/log/apache2/error.log) shows php fatal error as:

[Thu Nov 03 22:53:11.395812 2016] [:error] [pid 788] [client] PHP Fatal error:  Uncaught Doctrine\\DBAL\\DBALException: Failed to connect to the database: An exception occured in driver: SQLSTATE[HY000] [2002] No such file or directory in /data/web_data/nextcloud/lib/private/DB/Connection.php:64\nStack trace:\n#0 /data/web_data/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(429): OC\\DB\\Connection->connect()\n#1 /data/web_data/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(389): Doctrine\\DBAL\\Connection->getDatabasePlatformVersion()\n#2 /data/web_data/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(328): Doctrine\\DBAL\\Connection->detectDatabasePlatform()\n#3 /data/web_data/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(623): Doctrine\\DBAL\\Connection->getDatabasePlatform()\n#4 /data/web_data/nextcloud/lib/private/DB/Connection.php(151): Doctrine\\DBAL\\Connection->setTransactionIsolation(2)\n#5 /data/web_data/nextcloud/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/DriverManager.php(172): OC\\DB\\Connection->__construct(Array, Object( in /data/web_data/nextcloud/lib/private/DB/Connection.php on line 64

Mysql (Mariadb in my case) is working fine if accessed from mysql-client.

Nextcloud version : 15.0.04
Operating system and version : Linux 4.9.59-v7 on Raspberry Pi B
Apache or nginx version : 2.4.25
PHP version : 7.0.30-0+deb9u1

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

The output of your Nextcloud log in Admin > Logging: Not accessable

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

$CONFIG = array (
  'instanceid' => 'oca27i3xcegz',
  'passwordsalt' => 'u4gly9nTSKAiOeDru0FV43dVG8QhMQ',
  'secret' => 'fWub/quZ02XxStvFXYQ4BRu/kXPmKkblrHogT22NM1iZ/XPK',
  'trusted_domains' => 
  array (
    0 => '',
    1 => 'server',
  'datadirectory' => '/data/web_data/nextcloud_data',
  'overwrite.cli.url' => '',
  'dbtype' => 'mysql',
  'version' => '',
  'dbname' => 'nextcloud_db',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'installed' => true,
  'loglevel' => 0,
  'maintenance' => false,
  'theme' => '',
  'updater.secret' => '$2y$10$h8sP26T3e01hQFIXYLTxSusgl2B3y/EHqNMm6czWajnAb82Bz0E1.',

Looks like this file is missing. Did you do some backup previously?

Note: As you didn’t anomymised your config file as mentioned in the description, your instance is now compromised. Your instance secrets were published. Think about to do a fresh NC installation.

Kind regards