I set following option in /etc/httpd/conf.d/fcgid.conf file which solves the problem generally (but it would not when Plesk is installed):
OutputBufferSize 0
Try to configure OutputBufferSize option for the exact domain, for this create vhost.conf file
in domain document root (usually /var/www/vhost/domain.tld/conf folder), add OutputBufferSize
option with necessary value there and run
/usr/local/psa/admin/sbin/websrvmng -a -v