I have the same issue as here:
https://www.akeeba.com/support/admin-tools/Ticket/25881:0-class-fof30-container-container-not-found.html
When I try to login to the backend at https://mydomain.de/administrator/ or https://www.mydomain.de/administrator/ I get the error
Class 'FOF30\Container\Container' not found
I can however login through the subdomain that is also connected to the same root folder at https://relaunch.mydomain.de/administrator/