Wrong Minecraft version

Crash When Loading World: ClassNotFoundException: net.coderbot.iris.Iris

What the log looks like

java\.lang\.ClassNotFoundException: net\.coderbot\.iris\.Iris
java\.lang\.NoClassDefFoundError: net/coderbot/iris/Iris

How to fix it

  1. Look at the very first 'at ...' line directly underneath 'java.lang.NoClassDefFoundError: net/coderbot/iris/Iris' in your crash report to identify the outdated mod (for example, 'com.qendolin.betterclouds' means Better Clouds).
  2. Update that mod to its latest available release for your Minecraft version on Modrinth or CurseForge so it supports Iris 1.7+.
  3. If the crashing mod is Better Clouds, also update YetAnotherConfigLib (YACL) at the same time, as newer Better Clouds releases require a newer YACL version.
  4. If the offending mod has not been updated for Iris 1.7+ on your Minecraft version, either remove that mod from your mods folder or downgrade Iris (and Sodium, if required) to Iris 1.6.x.
LoadersFabric, Quilt, Neoforge
SeverityStops the game
CategoryWrong Minecraft version

Check your own log

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