There are PHP notices in Wordpress site, even though PHP error reporting was disabled or its level was set to
'E_ALL & ~E_NOTICE' in Parallels Plesk:
Notice: Undefined index: F1l3 in /var/www/vhosts/domain.tld/httpdocs/wp-includes/cache.php on line 2
Notice: Undefined index: F1l3 in /var/www/vhosts/domain.tld/httpdocs/class-wp-ajax-response.php on line 2 Notice
Wordpress has its own config which overrides virtual host PHP configs.
Disable error reporting in Wordpress config file the following way:
# grep DEBUG /var/www/vhosts/domain.tld/httpdocs/wp-config.php
See debugging in Wordpress for details.