I had this same issue of no back office when installing 1.7.7.0, until I updated my PHP7.2 to PHP7.4.
To do this:
Login to your hosting provider (Hostgator is mine) and go to the cPanel.
Software > MultiPHP Manager - Then select ea-php74 from the "PHP Version" dropdown menu.
Then just select your website from the available list below and press apply.
Afterwards I cleared my browser (Chrome) cache (CTRL + F5) and my browser history.
EVERYTHING IS NOW WORKING GREAT WITHOUT HAVING TO EDIT ANY CODE!