title: Wordpress - PHP Fatal error: Cannot access property WP_Hook::$callbacks
body: <p>Starting from yesterday (judging by the error logs), out of the blue, we notice the following error on the site:</p>
  
  <p>PHP Fatal error: Cannot access property WP_Hook::$callbacks in …/public_html/wp-includes/class-wp-hook.php on line 77</p>
  
  <p>The site is working (possibly thanks to the caching plugin), but all attempts to login to admin panel cause the error 500.</p>
  
  <p>We recovered the site from a backup from 6 of March (when the last successful login was), but the error persists.</p>
  
  <p>We also tried to deactivate all the plugins (by renaming the folders via FTP), but still the same error. (Now the work of plugins is recovered from a backup.)</p>
  
  <p>The hosting could not help to solve this problem.</p>
  
  <p>Where to look next?</p>
  
  <p>Site: <a href="http://russificateschool.com/" rel="nofollow noreferrer">http://russificateschool.com/</a></p>
  
  <p>Thank you in advance for the help.</p>