Are you guys serving with nginx? If so, please don't forget to write your admindir to the server config
# [EDIT] Replace 'admin-dev' in this block with the name of your admin directory. location /admin-dev/ { try_files $uri $uri/ /admin-dev/index.php$is_args$args; }
https://devdocs.prestashop-project.org/9/basics/installation/advanced/nginx/