Thank you both @tomp and @sdeziel!
It is strange that previously I never had issues running Docker and LXD on the same machine on Ubuntu 18.04, 20.04. Maybe it was the order I installed both frameworks?
ufw is deactivated on my system.
Unfortunately I was not able to restore network connectivity for LXC with the suggested iptables command. In my case I figured it should be:
sudo iptables -I DOCKER-USER -i lxdbr0 -o wlp0s20f3 -j ACCEPT
lxdbr0 is my LXD bride, wlp0s20f3 is my hardware WiFi interface.
LXC containers are still cut off from the internet.
Am I getting this right? (I’m sorry I don’t have much experience with iptables.)