Nextcloudpi: No video streaming // as external storage in second Nextcloud folder content doesn’t load

Nextcloudpi 17.0 on Raspberry Pi 4 (4GB) behind a Fritz!Box 7490 router:

Everything works fine with a dynamic DNS domain.

But inside the Nextcloud app it is not possible to watch videos or to listen to audio files. I’ve to download them offline first.
Same with another Nextcloud on a webserver. I set the Nextcloudpi as external storage. But especially in the app the files are not loaded.

Ports 80/443 are open to TCP/UDP and forwarded from the router. So the Nextcloudpi is reachable from the Internet.

So is there to open a further port?