13:33:18.257
Mixin prepare failed preparing net/minecraft/class_2960 in mixins.mm.json: org.spongepowered.asm.mixin.transformer.throwables.MixinTargetAlreadyLoadedException Critical problem: mixins.mm.json:net/minecraft/class_2960 target net.minecraft.class_2960 was loaded too early.
org.spongepowered.asm.mixin.transformer.throwables.MixinTargetAlreadyLoadedException: Critical problem: mixins.mm.json:net/minecraft/class_2960 target net.minecraft.class_2960 was loaded too early.
at org.spongepowered.asm.mixin.transformer.MixinInfo.readDeclaredTargets(MixinInfo.java:938)
at org.spongepowered.asm.mixin.transformer.MixinInfo.(MixinInfo.java:872)
at org.spongepowered.asm.mixin.transformer.MixinConfig.prepareMixins(MixinConfig.java:715)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:668)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:543)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:440)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:416)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:285)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:191)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:178)
at org.spongepowered.asm.mixin.transformer.FabricMixinTransformerProxy.transformClassBytes(FabricMixinTransformerProxy.java:23)
at net.fabricmc.loader.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:157)
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:148)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at net.fabricmc.loader.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:45)
at net.fabricmc.loader.EntrypointStorage$NewEntry.create(EntrypointStorage.java:114)
at net.fabricmc.loader.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:101)
at net.fabricmc.loader.EntrypointStorage.getEntrypointContainers(EntrypointStorage.java:186)
at net.fabricmc.loader.FabricLoader.getEntrypointContainers(FabricLoader.java:268)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:44)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:138)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)
13:33:18.345

Exception in thread "main" net.fabricmc.loader.api.EntrypointException: Exception while loading entries for entrypoint 'preLaunch' provided by 'optifabric'
at net.fabricmc.loader.EntrypointStorage.getEntrypointContainers(EntrypointStorage.java:193)
at net.fabricmc.loader.FabricLoader.getEntrypointContainers(FabricLoader.java:268)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:44)
at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36)
at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:138)
at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:26)
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [mixins.mm.json:net/minecraft/class_2960] from phase [DEFAULT] in config [mixins.mm.json] FAILED during PREPARE
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinError(MixinProcessor.java:643)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinPrepareError(MixinProcessor.java:591)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:545)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:440)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:416)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:285)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:191)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:178)
at org.spongepowered.asm.mixin.transformer.FabricMixinTransformerProxy.transformClassBytes(FabricMixinTransformerProxy.java:23)
at net.fabricmc.loader.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:157)
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:148)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at net.fabricmc.loader.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:45)
at net.fabricmc.loader.EntrypointStorage$NewEntry.create(EntrypointStorage.java:114)
at net.fabricmc.loader.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:101)
at net.fabricmc.loader.EntrypointStorage.getEntrypointContainers(EntrypointStorage.java:186)
... 5 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinTargetAlreadyLoadedException: Critical problem: mixins.mm.json:net/minecraft/class_2960 target net.minecraft.class_2960 was loaded too early.
at org.spongepowered.asm.mixin.transformer.MixinInfo.readDeclaredTargets(MixinInfo.java:938)
at org.spongepowered.asm.mixin.transformer.MixinInfo.(MixinInfo.java:872)
at org.spongepowered.asm.mixin.transformer.MixinConfig.prepareMixins(MixinConfig.java:715)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:668)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:543)
... 20 more
13:33:18.345
Process crashed with exit code 1

Source of IMBlocker: https://github.com/mrjesen/ImBlockerFabric

This is an issue in what IMBlocker is doing in its Mixin plugin. They're using the Minecraft class SharedConstants which is a big no-no, as it means no mod can apply Mixins to it.