Fixing 'Failed to login: Invalid session'

This message means Minecraft could not confirm the player's account with Mojang. It is almost always on the player's side rather than anything wrong with your server, and it usually clears in under a minute.

What the player should try, in order

1. Restart Minecraft completely, including the launcher, not just the game window. This refreshes the login session and fixes it most of the time.
2. Sign out and back in to their Microsoft account in the launcher.
3. Check Minecraft service status. If Mojang's login servers are having problems, everybody gets this at once until they recover, and there is nothing to fix at either end.

Other things that cause it

• The same account logged in twice. An account can only hold one session. If it is signed in on another device, or a previous session did not close cleanly, the new login is rejected.
• Rejoining too fast after a server restart. The client can still be holding the old session. Waiting a few seconds and reconnecting clears it.
• A launcher that has been open for days. Sessions expire. A launcher left running for a long time will eventually hand over a stale token, and a full restart is the fix.

When it is worth looking at the server

If everyone gets it at the same time and Mojang's status page is green, check whether your server is in offline mode. Session checks behave differently there and can produce confusing login errors. Unless you specifically need it, keep online mode on: see How to allow cracked players.