Minecraft Internal Exception Error
By Nathan Young

Encountering connection issues to a modded Minecraft server is an extremely common issue for players around the world. The causes can range from mismatched modlists to incompatibility. Keep in mind that this problem can occur on normal servers too. With this in mind, there’s one issue that has no obvious solution – frustrating users. This is called the Internal Exception Error, otherwise known as ‘io.netty.handler.codec.decoderexception’ or ‘java.io.IOEception’. This disrupts the connection to a Minecraft server, usually caused by client-sided problems. In this guide, we’ll be teaching you how to fix the Internal Exception Error!
What is the Internal Exception Error?
Joining a Vanilla or Modded Minecraft can sometimes return the following error: internal exception io.netty.handler.codec.decoderexception. There can be slight variations of this message, but it all has the internal exception part. This is caused by client-related issues with the Minecraft launcher or computer. However, it can also be a direct result of an issue on the dedicated server itself.

Regardless of the cause for Internal Exception Errors, troubleshooting and resolving them are the same. It’s important to note that if this situation impacts every player joining the Minecraft server, then this is highly likely an issue with the server. We strongly recommend contacting our Support Team if this is the case – we’ll help you fix the problem!
How to Fix the Internal Exception Error
There are many solutions to the internal exception io.netty.handler.codec.decoderexception error in Minecraft. These include updating Java, disabling VPNs, and many others. The following list will detail all of them, but more than one may be required to fix the situation.

- Update Java to its latest version on your computer
- Disable any VPN or Proxy masking your IP address
- Make an exception for Minecraft & Java in your Antivirus / Firewall
- Restart your Router / Connection
- Reinstall Minecraft or install in a new directory
- Allocate more memory to the Minecraft launcher
- Revert any recently made changes (uninstall mods, plugins, etc.)
- Enable the Java Native Sandbox option on your PC
- Remove any administrative restrictions on your network
- Restart the Minecraft server
- Reinstall mods or plugins on the Minecraft server
These solutions are considered the best to fix the Minecraft Internal Exception Error. Although, some of these may not entirely work for you – so make sure to try others until the problem is resolved. In cases where you are completely unable to fix the situation, consider reaching out to Mojang support if it’s confirmed that the Minecraft server is not responsible.
Minecraft Server Internal Exception Error
Challenges can arise when joining a Minecraft server, especially if it’s modded. The Internal Exception Error is one of these frustrating cases, leading players on a wild goose chase to find a solution. Fortunately, there are plenty of fixes for this problem – with some being better than others. It all depends on your specific situation, as they can have different causes. In cases where the Minecraft server won’t let anyone join, then it’s likely that the server itself has on-going issues. Regardless, we hope this article helped you fix the Minecraft Internal Exception Error!
FAQ – Internal Exception Error
This is a Minecraft Java error that disrupts any connections to a server, caused by various reasons. Players who encounter this issue must fix it on their client / computer.
You can fix this Minecraft error by updating Java, restarting your router, reverting any recently made changes, along with many others.
In cases where all players are unable to connect, then this is likely a Minecraft server related issue – only fixed by addressing the error(s) in the console. E.g. installing required mod dependencies, resolving world corruption, etc.
Useful Links
Minecraft Server Optimization Guide
How to Make an Offline Minecraft Server
How to Fix ‘Failed to Verify Username’ Error
How to Fix ‘Server Connection Timed Out’ Error