Server log:
[21:55:26] [User Authenticator #4/INFO]: UUID of player Cirkusapan is 203f749e-1a12-4bf9-86d2-5009d4cbbe9b
[21:55:26] [Server thread/INFO]: Cirkusapan[/192.168.2.10:60502] logged in with entity id 684 at (-0.5, 68.0, -1.5)
[21:55:26] [Server thread/INFO]: Cirkusapan joined the game
[21:55:27] [Server thread/INFO]: Cirkusapan lost connection: TranslatableComponent{key='disconnect.genericReason', args=[Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet], siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}

FML Log:
[21:55:26] [Server thread/INFO] [FML/]: Connection received without FML marker, assuming vanilla.
[21:55:26] [Server thread/INFO] [FML/]: [Server thread] Server side vanilla connection established