[BUG] io.netty.buffer.EmptyByteBuf.readByte(EmptyByteBuf.java:523)

One of the plugins has an issue when connecting a player from a browser (branded Eagler).
An "Internal Error" occurs on the client.

Velocity server logs:

[15:03:18 INFO] [limboapi]: Test (/127.0.0.1:50500) has connected to the LimboAuth Limbo
[15:03:18 ERROR]: java.lang.IndexOutOfBoundsException
[15:03:18 ERROR]: at io.netty.buffer.EmptyByteBuf.readByte(EmptyByteBuf.java:523)
[15:03:18 ERROR]: at net.lax1dude.eaglercraft.backend.server.base.pipeline.BufferUtils.readVarInt(BufferUtils.java:50)
[15:03:18 ERROR]: at net.lax1dude.eaglercraft.backend.server.base.handshake.VanillaInitializer.handleInbound(VanillaInitializer.java:107)
[15:03:18 ERROR]: at net.lax1dude.eaglercraft.backend.server.base.pipeline.WebSocketEaglerInitialHandler.encode(WebSocketEaglerInitialHandler.java:606)
[15:03:18 ERROR]: at net.lax1dude.eaglercraft.backend.server.base.pipeline.WebSocketEaglerInitialHandler.encode(WebSocketEaglerInitialHandler.java:51)
[15:03:18 ERROR]: at io.netty.handler.codec.MessageToMessageCodec$2.encode(MessageToMessageCodec.java:85)
[15:03:18 ERROR]: at io.netty.handler.codec.MessageToMessageEncoder.write(MessageToMessageEncoder.java:90)
[15:03:18 ERROR]: at io.netty.handler.codec.MessageToMessageCodec.write(MessageToMessageCodec.java:130)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:798)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:737)
[15:03:18 ERROR]: at io.netty.handler.timeout.IdleStateHandler.write(IdleStateHandler.java:306)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:798)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:737)
[15:03:18 ERROR]: at io.netty.handler.codec.MessageToByteEncoder.write(MessageToByteEncoder.java:116)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:800)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:737)
[15:03:18 ERROR]: at io.netty.handler.codec.MessageToByteEncoder.write(MessageToByteEncoder.java:120)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:800)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:737)
[15:03:18 ERROR]: at net.elytrium.limboapi.thirdparty.fastprepare.handler.PreparedPacketEncoder.write(PreparedPacketEncoder.java:66)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:800)
[15:03:18 ERROR]: at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:737)
[15:03:18 ERROR]: at io.netty.channel.DefaultChannelPipeline.write(DefaultChannelPipeline.java:1036)
[15:03:18 ERROR]: at io.netty.channel.Channel.write(Channel.java:267)
[15:03:18 ERROR]: at com.velocitypowered.proxy.connection.MinecraftConnection.delayedWrite(MinecraftConnection.java:253)
[15:03:18 ERROR]: at net.elytrium.limboapi.server.LimboImpl.onSpawn(LimboImpl.java:572)
[15:03:18 ERROR]: at net.elytrium.limboapi.server.LimboImpl.spawnPlayerLocal(LimboImpl.java:462)
[15:03:18 ERROR]: at net.elytrium.limboapi.server.LimboImpl.lambda$spawnPlayerLocal$1(LimboImpl.java:551)
[15:03:18 ERROR]: at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:148)
[15:03:18 ERROR]: at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:141)
[15:03:18 ERROR]: at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:535)
[15:03:18 ERROR]: at io.netty.channel.SingleThreadIoEventLoop.run(SingleThreadIoEventLoop.java:201)
[15:03:18 ERROR]: at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:1195)
[15:03:18 ERROR]: at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
[15:03:18 ERROR]: at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
[15:03:18 ERROR]: at java.base/java.lang.Thread.run(Thread.java:1583)
[15:03:18 INFO]: [connected player] Test (/127.0.0.1:50500) has disconnected: ?? ???????? ????????????? ?????????? ?????????????????? ??????? ???????!
[15:03:18 INFO] [limboapi]: Test (/127.0.0.1:50500) has disconnected from the LimboAuth Limbo
[15:03:18 INFO]: [initial connection] /127.0.0.1:50500 has disconnected

Also

[15:03:07 WARN]: The server channel initializer has been replaced by java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)

Velocity info:
velocity-3.5.0-SNAPSHOT-605

Velocity plugins:

[15:11:55 INFO]: Plugins: velocity, luckperms, maintenance, advancedserverlist, skinsrestorer, eaglerxserver, limboapi, limboauth, packetevents, pl-hide-pro, velocity-tools

Removing any plugin from the chain works, but if you use this specific combination:
limboapi
limboauth
packetevents

eaglerx breaks.

Start by tracing BufferUtils.readVarInt and VanillaInitializer.handleInbound in the stack trace, then inspect WebSocketEaglerInitialHandler.encode while reproducing the connection with limboapi, limboauth, packetevents, and eaglerxserver enabled. Compare the pipeline behavior with plugins removed; done means the browser connection no longer raises EmptyByteBuf.readByte or disconnects with an internal error.

Written by the indexing model from the issue text.