SECURE_AUTH_COOKIE error in WordPress Ghost plugin

When adding and activating a Ghost plugin, WordPress immediately kicks me out of the admin panel and throws this error.

Warning: Use of undefined constant SECURE_AUTH_COOKIE – assumed ‘SECURE_AUTH_COOKIE’ (this will throw an Error in a future version of PHP) in /var/www/wordpress/wp-includes/pluggable.php on line 766

Warning: session_start(): Cannot start session when headers already sent in /var/www/vendor/politsturm/wordpress-theme/admin/check_more.php on line 8
You do not have sufficient permissions to access this admin page.

When the plugin is removed via ftp, the error disappears.

Note: I am using WordPress Multisite

Does anyone have what to do? I don’t want to lose the article database when migrating to Ghost.