So, I tried to make my own NextCloud using this tutorial:
But when I run the following command (like he does at 3:53)
docker run --name nextcloud -d -p 7162:80 -v /home/pi/nextcloud:/var/www/html --network nextcloud-net nextcloud
I do get to the NextCloud page, but when I login with my PostGreSQL (like he does at 4:16), and I press continue, I get the following screen:
If you need to know anything else, please let me know.
Please use the search function of the forum and search for “Internal server error”. You will find many matches which all lead you e.g. to check the content of the Nextcloud log file and in most cases require to install/configure a missing php module 
I don’t know your setup but I got two suspicions…
Your Docker network-bridge is not setup correctly
The PostgreSQL container is not running, hence Nextcloud is unable to connect to it.
You can run the following commands to check the status of Docker and the containers:
Show running containers and their stats
docker stats
Show all container (running/not running)
docker container ls
Connect to a specific container
docker exec -it {container_name/id} /bin/bash
If you connect into the container, you can look at the logs (i.e. /var/log), because they usually give you a lot more information about the error and maybe allow users on here, to help you a little better.
Then what do I have to send from the /var/log folder?
Look for a nextcloud.log or a error_log - Those should contain something related to your internal server error
I dont see any of those files, I only see:
Could you tell me how to fix it? I would highly appriciate that.