https://paste.ubuntu.com/p/b4NpzdB3RG/ Server (ridx: 9, widx: 9, cap: 9)
https://paste.ubuntu.com/p/kh6wFrZN3j/ Client (Print by Mixin (ridx: 0, widx: 1, cap: 256)

forge-1.12.2-14.23.5.2847-universal.jar
JustEnoughIDs-1.0.3-52-nolibs.jar

+1, also occurs when I am playing LAN multiplayer, the computer that joins the game would crash when opening inventory

This is critical, please fix it.

There is a mixin on the client that is targeting CPacketCreativeInventoryAction, modifying the information sent such that it does not match the expectations of the server.

We have no mixins that target CPacketCreativeInventoryAction.

Without more information, I would have to conclude this is not a bug caused by this mod.

@sam-kirby It is. It's ok after downgraded.
PacketBuffer#readItemStack and writeItemStack had been modified in 52

@sam-kirby perhaps this issue should be reopened as its the first report of this happening? Dupes are #124 and #126