In online mode the server asks Mojang whether the person connecting really owns that account. Failed to verify username means it asked and did not get a usable answer — from the player's side, the server's side, or Mojang's.
What you are seeing
Failed to verify username!on connect.Authentication servers are down. Please try again later.Invalid session (Try restarting your game and the launcher).- One player affected while everyone else joins normally.
What to do
- Have the player fully restart the launcher
Not just the game — the launcher itself. Sessions expire, and the client holds a stale one until it is restarted. This alone fixes most single-player cases.
- Check whether it is everyone or one person
Everyone at once points at Mojang or at the server's outbound connectivity. One person points at their session or their account.
- Check Mojang's status
Authentication outages happen. There is nothing to fix on your side, and waiting is genuinely the correct action.
- Ask whether they are logged into the right account
A launcher signed into a second account, or a Microsoft account with a lapsed migration, produces this on a server it has never had trouble with before.
online-mode=false to make it go awayThat switch stops the server verifying anybody. Anyone can then connect under any username, including yours, with your operator permissions and access to your builds. It exists for backends behind a proxy that authenticates for them — nowhere else. If your server is directly reachable, this setting is the difference between a server and an unlocked door.
On a proxy network the proxy authenticates and the backends trust it. The backends run with online mode off and are unreachable from the internet. Both halves are required; doing the first without the second is the exploit, not the setup.
The affected player connects on the first attempt after a launcher restart, with no authentication line in the console.