Whenever I try to load 1.21 with Fabric Loader 0.16.0, I get this error message:
A mod crashed on startup!
net.fabricmc.loader.impl.FormattedException: java.lang.RuntimeException: Could not execute entrypoint stage 'preLaunch' due to errors, provided by 'debugify'!
at net.fabricmc.loader.impl.FormattedException.ofLocalized(FormattedException.java:63)
at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:162)
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:68)
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
Caused by: java.lang.RuntimeException: Could not execute entrypoint stage 'preLaunch' due to errors, provided by 'debugify'!
at net.fabricmc.loader.impl.FabricLoaderImpl.lambda$invokeEntrypoints$2(FabricLoaderImpl.java:403)
at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33)
at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:401)
at net.fabricmc.loader.impl.launch.knot.Knot.init(Knot.java:160)
... 2 more
Caused by: net.fabricmc.loader.api.EntrypointException: Exception while loading entries for entrypoint 'preLaunch' provided by 'debugify'
at net.fabricmc.loader.impl.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:56)
at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:399)
... 3 more
Caused by: java.lang.RuntimeException: Mixin transformation of com.llamalad7.mixinextras.MixinExtrasBootstrap failed
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:427)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.tryLoadClass(KnotClassDelegate.java:323)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:218)
at net.fabricmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:119)
at java.base/java.lang.ClassLoader.loadClass(Unknown Source)
at java.base/java.lang.Class.forName0(Native Method)
at java.base/java.lang.Class.forName(Unknown Source)
at java.base/java.lang.Class.forName(Unknown Source)
at net.fabricmc.loader.impl.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:50)
at net.fabricmc.loader.impl.entrypoint.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:117)
at net.fabricmc.loader.impl.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:53)
... 4 more
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [emi-fabric.mixins.json:conversion.ItemConvertibleMixin from mod emi] from phase [DEFAULT] in config [emi-fabric.mixins.json] FAILED during PREPARE
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinError(MixinProcessor.java:638)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinPrepareError(MixinProcessor.java:585)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:543)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:462)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:438)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:290)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:234)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:202)
at net.fabricmc.loader.impl.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:422)
... 14 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinTargetAlreadyLoadedException: Critical problem: emi-fabric.mixins.json:conversion.ItemConvertibleMixin from mod emi target net.minecraft.class_1935 was loaded too early.
at org.spongepowered.asm.mixin.transformer.MixinInfo.readDeclaredTargets(MixinInfo.java:947)
at org.spongepowered.asm.mixin.transformer.MixinInfo.(MixinInfo.java:881)
at org.spongepowered.asm.mixin.transformer.MixinConfig.prepareMixins(MixinConfig.java:933)
at org.spongepowered.asm.mixin.transformer.MixinConfig.prepare(MixinConfig.java:862)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:540)
... 20 more

It seems at first it is an issue with Debugify, but, it gives me the same error message with Controlify as well, and both times, removing EMI fixed the problem.

I found a fix for myselt by manually updating the Collective mod.
Its broken v7.79 was taken down, so I was unable to automatically update to the fixed v7.80 in the Modrinth App.
Maybe you have this mod too.

Having this issue too... emi was working before i updated all my mods using my launcher
EDIT: same exact class that was "loaded too early" and it blames another one of my mods too

Its an issue with conflicts, emi starts normally without any other mods and only jei & addons (and fabric api)
gonna comb through my mods list to see which one is causing this

Confirming EMI is broken

I also have that issue
latest.log

This is some mod loading classes too early, which seems to be the Collective mod on 7.79. Upgrading should resolve the issue