Fatal error after manual upgrade to 1.7.7.8

[LDTech]

Hi,
after following the official prestashop guide for manual upgrading (site url is https://kalamitica.com), I stopped just before doing the database upgrade because the back office freezed to a blank page, while the front office returns this error:

Quote

Warning: require_once(/home/customer/www/kalamitica.com/public_html/config/../vendor/autoload.php): failed to open stream: No such file or directory in /home/customer/www/kalamitica.com/public_html/config/autoload.php on line 26

Fatal error: require_once(): Failed opening required '/home/customer/www/kalamitica.com/public_html/config/../vendor/autoload.php' (include_path='.:/usr/local/php73/pear') in /home/customer/www/kalamitica.com/public_html/config/autoload.php on line 26

I've double checked the mentioned files and everything seems to be correct: paths are ok, filenames are ok (also for case sensitive), permissions are ok. Already removed the cache inside /var.
Also, the guide states that merging "vendor" folders could lead to issues and that it would be best to remove the old one; so I renamed it before the upgrade.
In other words, I followed the rules but I'm locked out.

Any ideas?

Edited May 10, 2022 by LDTech (see edit history)