This one is almost always the player's launcher rather than your server. The client could not prove to Mojang's session servers that it is who it says it is.
What you are seeing
Failed to login: Invalid session (Try restarting your game and the launcher)- It affects one player while others connect normally.
- It started after the player left the game open for a long time.
Why it happens
The launcher holds a session token that expires. When the client tries to join an online-mode server with a stale token, Mojang's authentication refuses it and the server declines the connection.
What to do
- Fully restart the game and the launcher
Not just the game — the launcher too. This refreshes the token and resolves it in the large majority of cases.
- Sign out and back in
If a restart is not enough, signing out of the Microsoft account in the launcher and back in forces a completely new session.
- Check whether the account is signed in elsewhere
The same account logged in on a second device can invalidate the first one's session.
The player connects normally. Nothing on the server side needs to change.
If every player gets this at the same moment, it is worth checking Mojang's own status — an authentication outage on their side looks exactly like this and no amount of restarting will help.