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