500 error URL Change

[CaptainBrillo]

My Prestashop is currently on :

amazing.com/shop (not the real name but it should be!)

When I enter: shop.amazing.com in a browser I come to: amazing.com/shop

When I change the shop URL in the backend to
URL: shop.amazing.com
SSL URL: shop.amazing.com
Base URL :/shop/

I enter: shop.amazing.com in a browser I get a 500 Error. in the frontend

And when I change the Base URL to: /

Prestashop backend breaks and the front end address is shop.xxxxxx.com/shop

Anyone got any ideas whats going on and how to solve it?