PHP Fatal error: require_once() [<a href='function.require'>function.require]: Failed opening required 'CRM/Core/Config.php' (include_path='.:/var/www/vhosts/www.xxxx/htdocs/modules/civicrm/:/var/www/vhosts/www.xxxx.net/htdocs/modules/civicrm//packages:.:/usr/share/pear') in /var/www/vhosts/www.xxxx.net/htdocs/sites/all/modules/civicrm/drupal/civicrm.module on line 224

Your civicrm_root is set to the wrong value. Set it to:

/var/www/vhosts/www.xxxx.net/htdocs/sites/all/modules/civicrm

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