Everyone's made one so ig I will too XD

[zinck78]

[23:26:39 WARN]: [MythicMobs] An error had occurred while modifying the packet ClientboundSetEntityDataPacket
[23:26:39 WARN]: java.lang.UnsupportedOperationException
[23:26:39 WARN]: at java.base/java.util.ImmutableCollections.uoe(Unknown Source)
[23:26:39 WARN]: at java.base/java.util.ImmutableCollections$AbstractImmutableList.set(Unknown Source)
[23:26:39 WARN]: at MythicMobsPremium-5.9.0-SNAPSHOT (8).jar//io.lumine.mythic.core.volatilecode.v1_21_R3.packets.VolatileChannelHandler.handleEntityDataPacket(VolatileChannelHandler.java:148)
[23:26:39 WARN]: at MythicMobsPremium-5.9.0-SNAPSHOT (8).jar//io.lumine.mythic.core.volatilecode.v1_21_R3.packets.PacketInterceptor$Modifier.modify(PacketInterceptor.java:49)
[23:26:39 WARN]: at MythicMobsPremium-5.9.0-SNAPSHOT (8).jar//io.lumine.mythic.core.volatilecode.v1_21_R3.packets.PacketInterceptor.accept(PacketInterceptor.java:34)
[23:26:39 WARN]: at MythicMobsPremium-5.9.0-SNAPSHOT (8).jar//io.lumine.mythic.core.volatilecode.v1_21_R3.packets.VolatileChannelHandler.write(VolatileChannelHandler.java:77)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:891)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:875)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:984)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:868)
[23:26:39 WARN]: at io.netty.channel.ChannelDuplexHandler.write(ChannelDuplexHandler.java:115)
[23:26:39 WARN]: at AxVaults-2.7.4.jar//com.artillexstudios.axvaults.libs.axapi.nms.v1_21_R3.packet.ChannelDuplexHandlerPacketListener.write(ChannelDuplexHandlerPacketListener.java:188)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:891)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:875)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:984)
[23:26:39 WARN]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:868)
[23:26:39 WARN]: at io.netty.channel.DefaultChannelPipeline.write(DefaultChannelPipeline.java:964)
[23:26:39 WARN]: at io.netty.channel.AbstractChannel.write(AbstractChannel.java:300)
[23:26:39 WARN]: at ProtocolLib (10).jar//com.comphenix.protocol.injector.netty.channel.NettyChannelProxy.write(NettyChannelProxy.java:210)
[23:26:39 WARN]: at ProtocolLib (10).jar//com.comphenix.protocol.injector.netty.channel.NettyChannelProxy.write(NettyChannelProxy.java:199)
[23:26:39 WARN]: at net.minecraft.network.Connection.doSendPacket(Connection.java:492)
[23:26:39 WARN]: at net.minecraft.network.Connection.lambda$sendPacket$13(Connection.java:479)
[23:26:39 WARN]: at ProtocolLib (10).jar//com.comphenix.protocol.injector.netty.channel.NettyEventLoopProxy.lambda$proxyRunnable$2(NettyEventLoopProxy.java:49)
[23:26:39 WARN]: at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:173)
[23:26:39 WARN]: at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:166)
[23:26:39 WARN]: at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:472)
[23:26:39 WARN]: at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:405)
[23:26:39 WARN]: at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
[23:26:39 WARN]: at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
[23:26:39 WARN]: at java.base/java.lang.Thread.run(Unknown Source)

1.21.4 is usable with protocol lib current for the most part. This is legit the only issue i'm experiencing with it. I'm sure you're hard at work developing the fix for this currently. But if you were unaware of the problem here's the snippet that i'm currently struggling with.