Call to undefined method FOF30\Platform\Joomla\Platform::getSessionVar()
Looking in your Ticket system I see the answer of dlb which is:
https://www.akeebabackup.com/support/akeeba-backup-3x/Ticket/27801-non-working-clean-3-7-1-website-after-install-of-akeeba-backup-pro-5-3-4.html
he says: "Copy all the files from the fof folder into your site's libraries/fof30 folder. WARNING! DO NOT copy the files into your site's fof folder."
I did that but the message in backend still appears.