That's stack trace in crash log:

java.lang.Error: Watchdog
at io.netty.channel.ChannelOutboundBuffer.addFlush(ChannelOutboundBuffer.java:155)
at io.netty.channel.AbstractChannel$AbstractUnsafe.flush(AbstractChannel.java:894)
at io.netty.channel.embedded.EmbeddedChannel$EmbeddedUnsafe$1.flush(EmbeddedChannel.java:894)
at io.netty.channel.DefaultChannelPipeline$HeadContext.flush(DefaultChannelPipeline.java:1372)
at io.netty.channel.AbstractChannelHandlerContext.invokeFlush0(AbstractChannelHandlerContext.java:750)
at io.netty.channel.AbstractChannelHandlerContext.invokeFlush(AbstractChannelHandlerContext.java:742)
at io.netty.channel.AbstractChannelHandlerContext.flush(AbstractChannelHandlerContext.java:728)
at io.netty.channel.DefaultChannelPipeline.flush(DefaultChannelPipeline.java:967)
at io.netty.channel.AbstractChannel.flush(AbstractChannel.java:254)
at net.minecraft.network.ClientConnection.doSendPacket(ClientConnection.java:3122)
at net.minecraft.network.ClientConnection.handler$dal000$krypton$sendImmediately$rewrite(ClientConnection.java:3065)
at net.minecraft.network.ClientConnection.sendImmediately(ClientConnection.java:199)
at net.minecraft.network.ClientConnection.send(ClientConnection.java:190)
at net.minecraft.server.network.ServerPlayNetworkHandler.sendPacket(ServerPlayNetworkHandler.java:1253)
at net.minecraft.server.network.ServerPlayerEntity.sendMessageToClient(ServerPlayerEntity.java:1369)
at net.minecraft.server.PlayerManager.broadcast(PlayerManager.java:813)
at net.minecraft.server.PlayerManager.broadcast(PlayerManager.java:805)
at net.minecraft.server.network.ServerPlayNetworkHandler.onDisconnected(ServerPlayNetworkHandler.java:1226)
at net.minecraft.network.ClientConnection.handleDisconnection(ClientConnection.java:452)
at net.minecraft.server.ServerNetworkIo.tick(ServerNetworkIo.java:185)
at net.minecraft.server.MinecraftServer.tickWorlds(MinecraftServer.java:908)
at net.minecraft.server.dedicated.MinecraftDedicatedServer.tickWorlds(MinecraftDedicatedServer.java:283)
at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:824)
at net.minecraft.server.MinecraftServer.handler$zho000$carpet$modifiedRunLoop(MinecraftServer.java:14942)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:650)
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:265)
at net.minecraft.server.MinecraftServer$$Lambda$7022/0x0000000801b640a8.run(Unknown Source)
at java.lang.Thread.run(Thread.java:833)

sounds like your server is lagging out and it just so happened that the server watchdog triggered while the server was trying to send a packet

Maybe from this? netty/netty#3938

That issue is caused by something neither vanilla or Krypton does

not from vanilla?

If you had really read the issue, it's caused by someone migrating channels between event loops. Neither vanilla or Krypton does this.

...so the issue is definitely not in Krypton.