When I try to connect to servers or join up on lan worlds in Minecraft it says "Failed to Login : Invalid session (Try restarting your game)". Multiplayer used to work fine but now keeps coming up with this error. Can anybody help?
Just as the error says your session ID, what you log in with, is invalid. This usually happens when you log in to your account and then have a new instance of minecraft and log in again without closing down the old one. The old session ID is invalidated as soon as the new connection is made. This will give you the error above when you attempt to log into a server as it will only have the new Session ID.
Restarting your client is the only way to get a new session ID as it will wipe out whatever it thinks the new one is and truly give you a new one. (Restart your client by quitting the game, closing and reopening the launcher, and then starting the game again)
If this problem persists, and you have already tried the possible causes (e.g. firewall, VPN), then try contacting Mojang customer support.