Problem/Bug Weird player kicking error

Code:

[15:49:51 WARN]: Failed to handle packet for /47.54.238.231:50360
java.lang.NullPointerException
at net.minecraft.network.NetHandlerPlayServer.func_147346_a(NetHandlerPlayServer.java:946) ~[nh.class:?]
at net.minecraft.network.play.client.C08PacketPlayerBlockPlacement.func_148833_a(SourceFile:60) ~[jo.class:?]
at net.minecraft.network.play.client.C08PacketPlayerBlockPlacement.func_148833_a(SourceFile:9) ~[jo.class:?]
at net.minecraft.network.NetworkManager.func_74428_b(NetworkManager.java:244) ~[ej.class:?]
at net.minecraft.network.NetworkSystem.func_151269_c(NetworkSystem.java:173) [nc.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:980) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:423) [lt.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:798) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:658) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:745) [?:1.7.0_72]
[15:49:51 INFO]: TheSaltySigner lost connection: Internal server error

This happens to some players, any idea what it could be?

 

Umm i think it means that a packet sent from the client cannot be handled by the server, is this server somehow protocol hacked?

 

what is protocol hacked?

 

@iSHFTx protocol hacked is a term used to describe a server that is for example version 1.7 and it has been tweaked to allow 1.8 clients in as well.

 

Its a pixelmon server running on 1.7.10

I found out it is the PC kicking players, so i dont think there is a solution

 

Moved to Bukkit Alternates.

 

Their Pixelmon may not be installed correctly. Pixelmon doesn't work properly on my desktop, it kicks me when I try to join a server. However, I have the same Forge build and Pixelmon installed on my Macbook and it works fine.