@pille, you’re in the wrong section…
Hello @pille,
welcome to the community of Nextcloud 
You placed your request in the general section, so it was moved to the support category. Unfortunately you ignored the template and a lot of information to help you is missing. Please edit your original post and add all required details like Nextcloud version, webserver type and version, os version, related log file content. Use the support template.
Without additional information the community members cannot help you.
Meanwhile, you’re looking for the default landing app in you config.php
occ config:system:set defaultapp --value="files"
try this but adapt to your install: How to edit config.php for Nextcloud snap