Enable error login in PHP:

ini_set('display_errors', 'on');
ini_set('log_errors', 'on');
ini_set('display_startup_errors', 'on');
ini_set('error_reporting', E_ALL);

Enable Osclass error log (file):

define('OSC_DEBUG', true);

Enable Osclass error log (database queries):

define('OSC_DEBUG_DB', true);

Both should be added into config.php file located in root folder of installation.