Solved Friends can't connect - IOException: Bad packet id 21

[xExekut3x]

• Ubuntu Server 14.04 - 3.13.0-30-generic kernel
• My owned dedicated server
• FTB Server version 1.6.4-965 - Monster 1.1.1
• No

java version "1.7.0_65"
Java(TM) SE Runtime Environment (build 1.7.0_65-b17)
Java HotSpot(TM) Client VM (build 24.65-b04, mixed mode)

CPU
http://pastebin.com/88DpNLd4
MEM
http://pastebin.com/ESxnnuQW
HDD
http://pastebin.com/zaJPufe8
Everything
http://pastebin.com/HCupQY7N

ForgeModLoader-server-0.log
https://www.dropbox.com/s/shso74s0765jjfn/ForgeModLoader-server-0.log
server.log
https://www.dropbox.com/s/p8j19agdhjc4a47/server.log

Relevant error
http://pastebin.com/uRFWrhaB

I can connect fine, both locally and through my public IP, but my friends cannot connect at all. They keep receiving the error I linked above. First the error occurred while I had Java 8 loaded, so I switched to Java 7 but received the same error. My friends can connect fine to a spigot server I have running.

I run the server with the default command:

java -Xms2048m -Xmx2048m -XXermSize=256m -jar FTBServer-1.6.4-965.jar nogui

Ha, it changed : P to a smiley.

My server.properties is
http://pastebin.com/xJynETmF

 

[xExekut3x]

I read somewhere that it might be a certain item causing the issue, and I did place a beacon down near the spawn. So I created a new world but still receiving the same error.

 

[xExekut3x]

Nevermind. My friends are stupid. They were using the regular MC launcher...