When you connect to the multiplayer lobby, 0ad uses a secure, TLS-encrypted connection by default. However, up to now we also offered the ability for players to disable that encryption. This won't be possible from some time tomorrow (2026-03-06) on anymore, as we want to improve the security and privacy of our players.
For the past two weeks, every player who disabled TLS-encryption in the past, and therefore connected to the multiplayer lobby with an unencrypted connection, received a notification to change their configuration to enable it. If you haven't followed these instructions or just haven't connected to the multiplayer lobby since then, here is what you have to do to continue to be able to connect to the multiplayer lobby:
From the main screen of the game, navigate to "Settings" → "Options" → "Networking / Lobby" and enable "TLS encryption" there.
As an alternative you can also locate your 0ad configuration file and change lobby.tls = "false" to lobby.tls = "true" there.