PHP Fatal error: apply_filters()

I run a lot of sites, all with iTS, and many of them sometimes log errors like this:

[27-Nov-2017 02:50:43] WARNING: [pool blah.org] child 5534 said into stderr: “Stack trace:”
[27-Nov-2017 02:50:43] WARNING: [pool blah.org] child 5534 said into stderr: “#0 {main}”
[27-Nov-2017 02:50:43] WARNING: [pool blah.org] child 5534 said into stderr: ” thrown in /var/www/vhosts/blah.org/httpdocs/afo-content/plugins/better-wp-security/better-wp-security.php on line 16″
[27-Nov-2017 02:50:43] WARNING: [pool blah.org] child 6189 said into stderr: “NOTICE: PHP message: PHP Fatal error: Uncaught Error: Call to undefined function apply_filters() in /var/www/vhosts/blah.org/httpdocs/afo-content/plugins/better-wp-security/better-wp-security.php:16”