I confirm this error happened to me too. The only thing that seems to be depending on is the day. I list a few things I did, although they did not help to the cause:
Closing and re opening the 0ad
Disconnecting and re connecting to the WIFI
different WIFI networks from different places
use (or not) of a VPN.
If there's two people, our experience is that: Person A hosts a game, but person B cannot join. Person B hosts a game, but person A cannot join. Usually, the way out of this is that people A & B join another third game (of a hypothetical person C). However, this can also result in problems more often than not.