java.lang.NoClassDefFoundError when generating/entering worlds. Only happening after porting modpack from Fabric to Quilt

[Lazs]

---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 2023-04-09 16:05:00
Description: Exception in server tick loop

java.lang.NoClassDefFoundError: net/fabricmc/fabric/mixin/object/builder/DefaultAttributeRegistryAccessor
	at net.nameplate.util.NameplateTracker.getMobLevel(NameplateTracker.java:37)
	at net.nameplate.util.NameplateTracker.startTracking(NameplateTracker.java:20)
	at net.minecraft.class_3231.handler$doj000$nameplate$startTrackingMixin(class_3231.java:3029)
	at net.minecraft.class_3231.method_18760(class_3231.java:222)
	at net.minecraft.class_3898$class_3208.method_18736(class_3898.java:1416)
	at net.minecraft.class_3898.method_18715(class_3898.java:1286)
	at net.minecraft.class_3898.method_17241(class_3898.java:906)
	at net.minecraft.class_3898.method_18714(class_3898.java:1070)
	at net.minecraft.class_3898.method_18701(class_3898.java:1202)
	at net.minecraft.class_3215.method_18755(class_3215.java:477)
	at net.minecraft.class_3218$class_5526.method_31436(class_3218.java:1560)
	at net.minecraft.class_3218$class_5526.method_31798(class_3218.java:1538)
	at net.minecraft.class_5579.method_31847(class_5579.java:217)
	at net.minecraft.class_5579.method_31820(class_5579.java:187)
	at net.minecraft.class_5579.method_31818(class_5579.java:168)
	at net.minecraft.class_3218.method_18771(class_3218.java:856)
	at net.minecraft.class_3218.method_18213(class_3218.java:842)
	at net.minecraft.class_3324.method_14570(class_3324.java:241)
	at net.minecraft.class_3248.method_33800(class_3248.java:151)
	at net.minecraft.class_3248.md27429f$polymer$lambda$polymer_prePlayHandshakeHackfest$0$1(class_3248.java:2552)
	at eu.pb4.polymer.impl.networking.EarlyConnectionMagic.lambda$handle$0(EarlyConnectionMagic.java:28)
	at eu.pb4.polymer.api.x.EarlyPlayNetworkHandler.lambda$onKeepAlive$0(EarlyPlayNetworkHandler.java:98)
	at net.minecraft.class_3738.run(class_3738.java:18)
	at net.minecraft.class_1255.method_18859(class_1255.java:157)
	at net.minecraft.class_4093.method_18859(class_4093.java:23)
	at net.minecraft.server.MinecraftServer.method_24306(MinecraftServer.java:776)
	at net.minecraft.server.MinecraftServer.method_18859(MinecraftServer.java:159)
	at net.minecraft.class_1255.method_16075(class_1255.java:131)
	at net.minecraft.server.MinecraftServer.method_20415(MinecraftServer.java:758)
	at net.minecraft.server.MinecraftServer.method_16075(MinecraftServer.java:752)
	at net.minecraft.class_1255.method_18857(class_1255.java:140)
	at net.minecraft.server.MinecraftServer.method_16208(MinecraftServer.java:737)
	at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:669)
	at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:257)
	at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.lang.ClassNotFoundException: net.fabricmc.fabric.mixin.object.builder.DefaultAttributeRegistryAccessor
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
	at org.quiltmc.loader.impl.launch.knot.KnotClassDelegate.loadClassOnly(KnotClassDelegate.java:178)
	at org.quiltmc.loader.impl.launch.knot.KnotClassDelegate.loadClass(KnotClassDelegate.java:150)
	at org.quiltmc.loader.impl.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:228)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
	... 35 more

[Gaming32]

What mods do you have installed? Or more specifically, do you have QSL/QFAPI installed?

[Lazs]

Here’s the full crashlog, including the modlist.