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This guide will help you solve the Unable to login: java.io.IOException with a 503 error type.
Note that we have a guide here for troubleshooting Minecraft errors which is explaining about this error too!
❓ What Is this Error ❓
If you have received a java.io.IOException with a 503 error, this means the Minecraft session servers are down.
How to Fix
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 into Mojang, and trying to connect again.
If this doesn’t work, the session servers 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 desperately need to get on your server, you can enable the offline mode.
Here is our guide explaining the offline mode and how to enable it.
If you find any issues with this error type, we ask you to contact us via live chat or the ticket system here.