Failed to login (invalid session)/Bad login
Error "Failed to login (invalid session)/Bad login." is usually caused when you're trying to login to the premium server with a non-premium account. If you don't own premium account, find another server. If you do own premium account and you're already logged in, try to close the game and re-join the server. If it still doesn't work check if Minecraft servers and services are online. If servers are down, take a break and come back later.
Check Minecraft services status (located on right side of page): Check now
Short answer - Possible problems:
- Non-premium user trying to join Premium server.
- The connection has expired and launcher needs to restart.
- Mojang/Minecraft servers are offline.