title: Site inaccessible after upgrading from 3.8.3 to 3.8.7
body: <p>The site auto update got to 100% but there was an error at the end.</p>
  
  <p>Now I cannot access the backend or the frondend and only get this error.</p>
  
  <p>Fatal error: Class 'Joomla\CMS\Plugin\CMSPlugin' not found in /home/.sites/92/site2697155/web/plugins/system/sessiongc/sessiongc.php on line 23</p>
  
  <p>Can someone help me to restore my site?</p>