500 and 403 error after migration

Hi,

I made a website for a company and I wanted to put a back-up version of the site on my server in order to tests somes things instead of doing it on the live site. So I made a back-up of the db (using phpmyadmin), made a back-up of the files, imported the db in a new db on my server, imported the files, then changed the wp-config file so it connects to the new db, when in the db (via phpmyadmin) to change the url of the site and home.

The problem now, is that when I go to the new url, I see the home page, but if I click on the menu for another page, I got a 500 error. And if I try to go to the wp-admin, I got a 403 error.

So, I checked the permissions (all set to 755), deleted the .htaccees file, desactivated the plugins… nothing works…

I lookedat many posts, but it always seems to related to the htaccess file, the plugins or the permissions. Do I have to change something else in the db?