The issue appears to be related to the Members Plugin. After deactivating it, the website functionality was restored. I also installed this plugin on one of our staging environments to better diagnose the error. The log file provided me with a PHP fatal error that is linked to the Members Plugin. The error message is as follows:
“PHP Fatal error: Uncaught Error: Call to undefined function Members\Admin\members_is_admin_page() in /var/www/vhosts/stage01.(……)/httpdocs/wp-content/plugins/members/admin/class-notifications.php:436”
This error suggests that there is a problem with a function called “members_is_admin_page()” in the Members Plugin. This function seems to be undefined or not accessible at the given location, which is causing the fatal error.