Thank you for your quick reply.
Firstly I formated my cache drives. Then I shut the server down and backed up the USB-drive. Pluged into my PC I deleted the content of the config folder. I only left the KEY file in there.
Then I put it back in the server. The boot runs through, I can see the assigend IP-Adress on the screen. But going to the IP adress I cannot connect to the server. What could went wrong? When I load all the files back on the USB drive the server starts. But I still have the issues with Nextcloud and MariaDB.