There is the green checkmark and I tried it with Disable certificate verification (insecure) without success.
There is an optional reverse proxy. There is the following situation. There are two server in my flat. The first is the main server, which listen on port 80 and 443. The second, where this Nextcloud instance is running, listen on port 9300. Therefore there are two possible domains, to access the nextcloud. There is a domain, which points direct to the port 9300 of the Nextcloud server (second) and there is a port 80/443 sub domain, which points the main server and redirect to the port 9300 sub domain. I uses the direct domain to avoid problems with the reverse proxy.
The main server create all certificates via letsencrypt and send the required certificates to the second server. On the main server there is a collobara container running, which is used by nextcloud instances of the second server and also by a nextcloud instance running on the main server. Before I restored the backup, it worked well and collabora is still working on the instance on the main server.