Description of my issue:
Hello,
After upgrading to Akeeba Backup 8 I got the "FOF40\Container\Container' not found" error on two out of 18 sites but, despite uploading the fof40 folder and resetting the Opcache the problem was not resolved. (Same as described here: https://www.akeeba.com/support/akeeba-backup-3x/Ticket/34649:class-fof40-container-container-not-found.html)
What did fix it for me was to delete the plugins/system/backuponupdate folder (which allowed me back into the site admin area) then reinstall Akeebabackup 8 twice. (I saw a similar ticket referring to the fof30 folder which led me to try this.)