Based on 1.11.2 decompiled using MCP 9.35 rc1

The bug

The constructor for the playerlist header and footer packet (SPacketPlayerListHeaderFooter) is missing a footer parameter. This is not a problem in vanilla gameplay because this packet is currently not send, but will very likely cause problems in the future.

The problem is that like this you cannot set a footer text component and therefor when the packet is sent the server tries to turn null into a JSON String which is not possible and results in the connection to the client being closed.

In 17w06a the obfuscated name of the packet class is ih.