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Error fix

java.util.ConcurrentModificationException

Learn how to resolve the java.util.ConcurrentModificationException in your Minecraft server. Understand the causes and how to fix it with our guide.

AwakeNode Team · Reviewed by Harry · Updated 15 Oct 2026

What is ConcurrentModificationException?

The java.util.ConcurrentModificationException occurs when a thread attempts to modify a collection (such as a list or map) while another process is actively iterating over it. In the context of a Minecraft server, this usually indicates that a plugin or mod is trying to change data in a way that conflicts with the server's main tick loop.

Common Causes

  • Incompatible Plugins: Two or more plugins attempting to modify the same player data or entity list simultaneously.
  • Outdated Software: Using a plugin version that is not compatible with your current server software version.
  • Asynchronous Tasks: A plugin trying to modify server objects from an asynchronous thread instead of the main server thread.

How to fix it

On AwakeNode

  1. Use the File Manager: Navigate to your server's plugins folder via the AwakeNode File Manager to identify recently added plugins.
  2. Isolate the Cause: Remove or disable suspected plugins one by one. Restart your server after each change to see if the error persists in the console.
  3. Contact Support: If the error continues after removing plugins, open a ticket with our support team. We can review your crash logs and server environment to determine if a specific startup flag or configuration adjustment is required.

On other hosts / manually

  1. Check Logs: Access your latest.log or crash-reports via SFTP. Look for the stack trace immediately following the ConcurrentModificationException to identify which plugin or mod is triggering the conflict.
  2. Update or Remove: Ensure all plugins and mods are updated to the latest stable release. If the error persists, remove the plugin identified in the stack trace.
  3. Adjust Startup Flags: If the error is caused by improper memory handling or threading issues, you may need to edit your startup command. Locate your server's startup script and ensure your -Xmx and -Xms flags are set correctly for your RAM allocation. If you are unsure how to configure these, consult your host's documentation.

Questions

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AwakeNode is a trading name of Falcon Hosting Ltd.
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Will this error crash my server?+

It depends on the severity. While it often logs as a warning, it can cause a server crash if the modification conflict occurs on a critical data structure.

Can I fix this by adding more RAM?+

No. This error is caused by code conflicts, not a lack of memory. Upgrading your plan will not resolve a ConcurrentModificationException.

How do I know which plugin is causing the error?+

Check your server logs for the stack trace. The name of the plugin or mod is usually listed in the lines immediately following the exception message.

Should I change my Java version?+

Generally, no. Ensure you are using the Java version recommended for your specific Minecraft version, but this error is almost always plugin-related.

More error fixes

java.lang.NoClassDefFoundError / ClassNotFoundExceptionLearn how to resolve java.lang.NoClassDefFoundError and ClassNotFoundException errors on your Minecraft server with our step-by-step troubleshooting guide.java.lang.NoSuchMethodError / NoSuchFieldErrorLearn how to resolve java.lang.NoSuchMethodError and NoSuchFieldError on your Minecraft server by identifying version mismatches and plugin conflicts.Exception in server tick loopLearn how to diagnose and resolve the 'Exception in server tick loop' error on your Minecraft server with our step-by-step troubleshooting guide.java.net.SocketException: Connection resetTroubleshoot the java.net.SocketException: Connection reset error on your Minecraft server. Learn how to diagnose network and client-side issues.

On this page

  • What is ConcurrentModificationException?
  • Common Causes
  • How to fix it

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