I upgraded from ownCloud 9 to Nextcloud 11 a few weeks ago and ever since, I've been unable to get the mobile app to work for Android. After a few days I began to realise that the app was trying to connect on IPv6, but failing. Removing my AAAA record ensured that the app connected using IPv4, but I was perplexed and wanted to get to the bottom of the problem...
After loads of pinging and tracerouting, I figured that any LXC VMs I had on my Raspberry Pi were simply not working on IPv6. I was able to ping out with them, but I couldn't ping to them.
After tearing my hair out for a while, I finally got to the bottom of it. You need to enter the following into a shell terminal:
sudo sysctl net.ipv6.conf.br0.disable_ipv6=0
I'm too afraid to reboot it, but I hope that it's persistent!