Media Library huge issue: files exist but not showing both FE and BE

Hi. I’m having big issues with media library!
I need to update and clean a huge site which has not been updated since a couple of years..
I’ve just migrated the real site to our test space, I’ve replaced all old urls with the test ones and changed the wp-config.php to direct to the test db.

All the operations went fine, but both from front-end and back-end I cannot see uploaded files:

even if their paths are correct

and they actually exist

Unfortunately our test site is on our office vpn, this is the reason why I posted you images instead of a link, but you can see the production site here: Piccole Figlie

ps. a very strange thing happened while I was taking screenshot for this message. After searching the images in the browser by url, I noticed that, refreshing the frontend test page, images appeared…

Still nothing in the backend though..

Could you please help me? Of course I cannot search for each image in such a gigantic website and anyway I need to start my updating tests with the same conditions as the real website.
Thanks for helping

Marta