Hi. CatServer 1.12.2 / CleanRoom 0.0.30. When trying to launch AE2uel, a more recent version crashes the client with the error mixinbooter.

Description: Initializing game

java.lang.NoClassDefFoundError: scala/collection/Seq
at java.base/java.lang.Class.getDeclaredMethods0(Native Method)
at java.base/java.lang.Class.privateGetDeclaredMethods(Unknown Source)
at java.base/java.lang.Class.getDeclaredMethods(Unknown Source)
at net.minecraftforge.fml.common.FMLModContainer.gatherAnnotations(FMLModContainer.java:402)
at net.minecraftforge.fml.common.FMLModContainer.constructMod(FMLModContainer.java:597)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)
at java.base/java.lang.reflect.Method.invoke(Unknown Source)
at LAUNCHER//com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:85)
at LAUNCHER//com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:142)
at LAUNCHER//com.google.common.eventbus.Subscriber.lambda$dispatchEvent$0(Subscriber.java:71)
at LAUNCHER//com.google.common.util.concurrent.DirectExecutor.execute(DirectExecutor.java:31)
at LAUNCHER//com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:68)
at LAUNCHER//com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:111)
at LAUNCHER//com.google.common.eventbus.EventBus.post(EventBus.java:263)
at net.minecraftforge.fml.common.LoadController.sendEventToModContainer(LoadController.java:317)
at net.minecraftforge.fml.common.LoadController.propogateStateMessage(LoadController.java:295)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)
at java.base/java.lang.reflect.Method.invoke(Unknown Source)
at LAUNCHER//com.google.common.eventbus.Subscriber.invokeSubscriberMethod(Subscriber.java:85)
at LAUNCHER//com.google.common.eventbus.Subscriber$SynchronizedSubscriber.invokeSubscriberMethod(Subscriber.java:142)
at LAUNCHER//com.google.common.eventbus.Subscriber.lambda$dispatchEvent$0(Subscriber.java:71)
at LAUNCHER//com.google.common.util.concurrent.DirectExecutor.execute(DirectExecutor.java:31)
at LAUNCHER//com.google.common.eventbus.Subscriber.dispatchEvent(Subscriber.java:68)
at LAUNCHER//com.google.common.eventbus.Dispatcher$PerThreadQueuedDispatcher.dispatch(Dispatcher.java:111)
at LAUNCHER//com.google.common.eventbus.EventBus.post(EventBus.java:263)
at net.minecraftforge.fml.common.LoadController.distributeStateMessage(LoadController.java:231)
at net.minecraftforge.fml.common.Loader.loadMods(Loader.java:600)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:232)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:467)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:378)
at net.minecraft.client.main.Main.main(SourceFile:123)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)
at java.base/java.lang.reflect.Method.invoke(Unknown Source)
at top.outlands.foundation.LaunchHandler.launch(LaunchHandler.java:119)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Unknown Source)
at java.base/java.lang.reflect.Method.invoke(Unknown Source)
at LAUNCHER//top.outlands.foundation.boot.Foundation.main(Foundation.java:41)
at java.base/java.lang.invoke.MethodHandle.invokeWithArguments(Unknown Source)
at pro.gravit.utils.launch.ModuleLaunch.launch(Unknown Source)
at pro.gravit.launcher.fAntAsYWORldF6.fAntaSywORLdcM(Unknown Source)
at pro.gravit.launcher.fAntAsYWORldF6.fAntaSywORLdcM(Unknown Source)
at pro.gravit.launcher.fAntAsYWORldF6.main(Unknown Source)
Caused by: java.lang.ClassNotFoundException: scala.collection.Seq
at LAUNCHER//top.outlands.foundation.boot.ActualClassLoader.findClass(ActualClassLoader.java:237)
at LAUNCHER//top.outlands.foundation.boot.ActualClassLoader.loadClass(ActualClassLoader.java:244)
... 42 more
Caused by: java.lang.ClassNotFoundException: scala.collection.Seq
at LAUNCHER//top.outlands.foundation.boot.ActualClassLoader.findClass(ActualClassLoader.java:208)
... 43 more
No Mixin Metadata is found in the Stacktrace.

Since you're using Cleanroom, did you remember to install the mods Scalar and Fugue? You should also disable Mixinbooter if the pack you're playing came with the mod as Cleanroom natively handles mixins.