Fatal Error after trying to upgrade PS to last version (ComposerAutoloader line 7) (SOLVED)

[PinLoco]

Hello to all
A friend was trying to update his prestashop store site from 1.7.7.1 to the latest version (1.7.8.3) with "1 click upgrade" but now the site is unusable with the following error.

Fatal error: Uncaught Error: Class 'ComposerAutoloaderInit8d981fb0e3c83553e712cea0b1387cd2' not found in /homepages/15/d370206513/htdocs/vendor/autoload.php:7 Stack trace: #0 /homepages/15/d370206513/htdocs/config/autoload.php(26): require_once() #1 /homepages/15/d370206513/htdocs/config/config.inc.php(38): require_once('/homepages/15/d...') #2 /homepages/15/d370206513/htdocs/admin6492/index.php(40): require('/homepages/15/d...') #3 {main} thrown in /homepages/15/d370206513/htdocs/vendor/autoload.php on line 7

He asked me for help, and although I use other CMS but I have never worked with prestashop. I'm seeing where the error comes from (autoload.php) and when I downloaded the Prestashop 1.7.8.3 package I found that the content of the autoload on my friend's website and the one in the package is the same.

We only have access to the website content via ftp (I don't know if it is possible to update Prestashop by replacing files as it is done in wordpress).
The PHP version of the server is 7.3 but easily upgradable to 7.4 (not upgraded before for compatibility with Prestashop version 1.7.7.1 being used until now).

edit: Changed to 7.4 and everything is the same

We have the option to load backups and start the upgrade again from them

Thanks in advance.

Edited January 31, 2022 by PinLoco
more info. Solved (see edit history)

[PinLoco]

update:

We have upgraded PHP to 7.4 and then following the upgrade tutorial manually we have managed to get the site working (although we got a small error when updating the DB, which when reinstalling gave us the answer "28" (it was already installed)).
Now the page is a bit slow, but it seems to be working, we will try to optimize DB or install a cache plugin.
If anyone has any other idea is welcome.
Thanks to all and I hope that if it happens to someone the same as my friend can see that manually installing and upgrading to PHP 7.4 seems to fix it.

 

Edit 2: 

We have found new issue with orders, but solved following 

https://github.com/PrestaShop/PrestaShop/issues/22215

 

Edited January 31, 2022 by PinLoco (see edit history)