Magento CE 1.9.0.1 – Array to String Conversion Zend DB Adapter Pdo Abstract.php

I get this Error in Magento… /var/log/system.log since
a few weeks, – CE 1.9.0.1 with Firegento, german localisation Mage_Setup

  ERR (3): Notice: Array to string conversion  in
...\htdocs\magento\lib\Zend\Db\Adapter\Pdo\Abstract.php on line 74

I don´t know what does it mean. The code

protected function _dsn()
{   // baseline of DSN parts
    $dsn = $this->_config;

    // don't pass the username, password, charset, 
            // persistent and driver_options in the DSN
    unset($dsn['username']);
    unset($dsn['password']);
    unset($dsn['options']);
    unset($dsn['charset']);
    unset($dsn['persistent']);
    unset($dsn['driver_options']);

    // use all remaining parts in the DSN
    foreach ($dsn as $key => $val) 
            {
        $dsn[$key] = "$key=$val";  /* this is line 74 */
    }
    return $this->_pdoType . ':' . implode(';', $dsn);
}

What do I have to do.
Many Thanks for help, I didn´t find any help and answers in the web, in magento forums.

Petra Klapproth
(localhost for testing)