Error: require_once(): Failed opening required 'CRM/Core/ClassLoader.php'
I'm using drupal with civiCRM and got the same error [failed to open require_once CRM/Core/ClassLoader.php].
I made the following changes to solve it.
In your civicrm.settings.php if you have
require_once 'CRM/Core/ClassLoader.php';

Change it by adding &quot;civicrm_root&quot; variable
require_once $civicrm_root . '/CRM/Core/ClassLoader.php';

This is assuming that somewhere in that file, you have already set the &quot;$civicrm_root&quot; variable to point to your civiCRM install directory such as
$civicrm_root = '/var/www/html/drupal-root/vendor/civicrm/civicrm-core';

Note: my path might be different as I'm using composer to install civiCRM