I did it like you adviced and it worked... I figuered out the difference this time was that I used the zip file with etcher. So I just zipped the ISO of nextcloud and it works... Very strange. Thanks for the help, but I still would like to know why it only works with zip files and not the ISO itself.

Good morning together, I bought myself a raspberyy pi to use it as a nextcloud. I also ordered a case with active and passive cooling. It has a PSU which fits the Pi as well. I follow a 3 months old Video from Youtube to get the pi and the nextcloud started: https://www.youtube.com/watch?v=KLNl2uBGU...