Launcher problems

NoClassDefFoundError: org/apache/logging/log4j/LogManager

What it looks like in the log

java.lang.NoClassDefFoundError: org/apache/logging/log4j/LogManager

How to fix it

  1. Create and launch a plain vanilla profile for the exact same Minecraft version once so the launcher downloads all required base libraries.
  2. Update your third-party launcher (such as HMCL, GDLauncher, or Prism Launcher) to its latest release, then recreate the modded instance.
  3. Delete the `org/apache/logging/log4j` folder inside your launcher's `libraries` directory and launch the game while connected to the internet to force a fresh download.
  4. If every instance in your launcher fails with this error, back up your worlds/instances and reinstall the launcher cleanly to reset its broken library index.
LoadersFabric, Forge, Vanilla
SeverityStops the game
CategoryLauncher problems

Check your own log

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