I tried multiple things, as using lighttpd or nginx. I messed around with all the caching settings, ssl and any options i could think of. It just didnt work. I think the dedicated Ubuntu Server image for Rock Pi 4 is just messed up.
I now have the same setup on a newly purchased Intel Nuc running stock Ubuntu Server 18.04.02 LTS and it works flawlessly. So I can’t recommend the Rock Pi for Nextcloud unfortunately.