Hey @pille welcome to the Nextcloud community 
You placed you 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 probably looking for LDAP see User authentication with LDAP — Nextcloud latest Administration Manual latest documentation