Hello from 2024.
I rebooted my server today and I was seeing the
Error response from daemon: network br0 not found Error: failed to start containers: <container name>
error like OP was.
Somehow I didn't have a static IP set in my Unraid OS settings. So going Settings > Network Settings and changing my IPv4 to a static address worked. To change the setting I had to turn off the Docker service, so when I turned on the Docker service again all of my containers came back up immediately. No need to run any commands for me, apparently.