Wrong Minecraft version

NoClassDefFoundError: org/apache/logging/log4j/util/ReflectionUtil

What it looks like in the log

java.lang.NoClassDefFoundError: org/apache/logging/log4j/util/ReflectionUtil

How to fix it

  1. Check the stack trace lines immediately under `NoClassDefFoundError: org/apache/logging/log4j/util/ReflectionUtil` to identify the mod calling `Log4jLoggerFactory` (almost always `com.sk89q.worldedit` for WorldEdit).
  2. Download and install the latest version of that mod for your Minecraft version from Modrinth or CurseForge—for WorldEdit on Minecraft 1.16.3 to 1.16.5, update to version 7.2.5 or newer, and delete the old JAR from your `mods` folder.
  3. If you are playing an older Minecraft version (such as 1.15.2) and already have the newest release on CurseForge, check the mod author's official build server (such as EngineHub builds for WorldEdit) for a newer maintenance build, or remove the mod.
  4. Do not manually downgrade your launcher's `log4j-api` or `log4j-core` JARs back to 2.8.1 to bypass this error, as doing so exposes your client or server to the Log4Shell remote code execution exploit.
LoadersFabric, Forge, Quilt, Server
SeverityStops the game
CategoryWrong Minecraft version

Check your own log

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