@drcoolzic and another post got me headed in the right direction as I started my HA journey on a RPi4 Ubuntu arm64 node.
Unfortunately was sitting getting errors - for anyone still struggling (I’m doing everything locally, not externally - yet), I had to adjust my Ubuntu firewall
Instead of:
ufw allow from <subnet>/<mask> to any port 8123
Using this cleared it right up:
ufw allow from any to any port 8123
As I’m using a manual supervisor install (thus docker), I believe the chatter was being cut off. I plan to dig a bit deeper to determine what is being called/used so I can lock down a bit tighter but hoping this helps others.