In plesk in comination for PHP, the error reporting is redirected to a log file, and instead a Whitescreen is shown. To change that, e.g. for a development-system:
echo "php_admin_flag display_errors on
php_admin_value error_reporting 2047" >> conf/vhost.conf
/usr/local/psa/admin/sbin/websrvmng -u --vhost-name=DOMAIN
Applies for domains and subdomains as well.
Reposted from notes.it-jobs-und-stellen.de