If you have received a java.io.IOException with a 503 error, this means the Minecraft session servers are down.

Sometimes this is a glitch that can be fixed by:

Closing your game.
Logging-out of your launcher.
Restarting the server.
Restarting your launcher, logging back in to Mojang and trying to connect again.

If this doesn't work, the session servers really are down, and you’ll be unable to join any server with online mode enabled. Most servers have online mode enabled for security. If you are desperate to get on your server, you can disable online mode: How To: Change your server to offline mode
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