PHP errors with WordPress 4.9.x

Hello, after upgrading to WordPress 4.9 (tested on 4.9.2 and 4.9.5) I’ve started seeing the following PHP errors repeatedly on every page load:

[09-Apr-2018 20:07:25] WARNING: [pool wordpress] child 1163 said into stderr: “NOTICE: PHP message: PHP Warning: fopen(): Filename cannot be empty in /opt/wordpress/releases/6961904477ab2b7a53374326a07cb819/wp-includes/functions.php on line 4834”
[09-Apr-2018 20:07:25] WARNING: [pool wordpress] child 1163 said into stderr: “NOTICE: PHP message: PHP Warning: fread() expects parameter 1 to be resource, boolean given in /opt/wordpress/releases/6961904477ab2b7a53374326a07cb819/wp-includes/functions.php on line 4837”
[09-Apr-2018 20:07:25] WARNING: [pool wordpress] child 1163 said into stderr: “NOTICE: PHP message: PHP Warning: fclose() expects parameter 1 to be resource, boolean given in /opt/wordpress/releases/6961904477ab2b7a53374326a07cb819/wp-includes/functions.php on line 4840”

I was able to isolate these errors to the Cornerstone 0.7.5 plugin, after disabling all other plugins.