This error appears when the server tries to verify a player with Mojang but cannot. The server and client must agree on authentication.

Common causes and fixes

1. Cracked or non-premium launcher. online-mode is true, which only allows real accounts. Use a legitimate account, or see Online Mode Explained.
2. Mojang session servers are down or slow. Wait a few minutes and try again, then restart Minecraft.
3. Player was already logged in. Fully close the game and try again, or rejoin after a minute.
4. Behind a proxy. Proxy setups need the backend server to use online-mode=false and the proxy to forward player info. See Velocity.
5. Bad connection. Try a different network or restart your router.

Quick fix for the server owner

Check the console for any messages when the player joins, then confirm online-mode in server.properties.

Still stuck? Share your log (see Share Your Server Logs) in a support ticket.