Hello @falko, you started a topic in support category.
Unfortunately you ignored the template and a lot of information to help you is missing.
Please add all necessary information like Nextcloud version, webserver type and version, os version, related log file content.
Without additional information the community members cannot help you.
It sounds you question relates to CODE itself - in this case you better ask in Collabora Forum. You could also use a reverse proxy with integrated support of automatic certificate renewal like caddy traefik 101: reverse proxy
Regards,
wwe